How Can you Make a VHF TV Antenna for an Attic

Hi - I need to receive VHF TV (channels 6,7,9,13) and would like to
make a super-duper antenna for inside my attic. I would have thought
that I could easily find (simple) instructions on the internet but
can't. Does anybody have a simple idea that just uses wire (wire
should be easy to attach in an attic).

I've seen some instructions (mostly UHF or DTV) and some of them do
calculations for wavelength (let's say 5 feet). And then, with no
explanation, the guy just says "I made it 10 feet for better
reception". So I ask, can I not then just use the entire length of my
attic for super-duper reception? Wire is cheap after all, and I only
want to crawl up there once.

I don;t have a PhD in antenna making, so a lot of the instructions/
terms don't mean much to me (dipole, balun, etc). I'm hoping for
instructions such as:
1. Cut a piece of 18gauge coppr wire 5 feet long
2. attach one end to a rafter.
3. solder the other end to the centre wire of the coax
4 insert tab A into slot B
etc
etc.


Also, I see instructions that say you should aim the antenna without
defining "aim". Do you allign the wire in the direction of the
transmission antenna, or should the wire by perpendicular?

Thanks

 

[Sal M. Onella]

<wdoe999@yahoo.com> wrote in message
news:fd0a86cc-2039-485c-b8a5-243f7b7bff03@m11g2000yqh.googlegroups.com...

Hi - I need to receive VHF TV (channels 6,7,9,13) and would like to
make a super-duper antenna for inside my attic. I would have thought
that I could easily find (simple) instructions on the internet but
can't. Does anybody have a simple idea that just uses wire (wire
should be easy to attach in an attic).

I'd like the answer to be YES, but it's NO.. If you wanted to make an
antenna for just one channel, I'd say yes. I already did it for some guy
who wanted a channel 2 antenna over a year ago.

I've seen some instructions (mostly UHF or DTV) and some of them do
calculations for wavelength (let's say 5 feet).

It's a multi-step process. You have to look up the channel frequency for a
TV channel. Then, you take the number 300 and divide it by the frequency.
The result is the wavelength. The elements are then cut for approximately a
half-wavelength. More details below, if you want 'em.

Multi-channel antennas have multiple elements, all differing lengths. If
you have one element, you can expect to receive one channel well and other
channels maybe but not as well. A single channel antenna can be made of TV
twinlead and attached to a piece of wood. It's called a "folded dipole."
More below.

And then, with no
explanation, the guy just says "I made it 10 feet for better
reception". So I ask, can I not then just use the entire length of my
attic for super-duper reception?

Nope. He's full of it to say that. The only thing that gets longer to make
a better antenna is the boom, the center long rod of a long antenna, and it
gets longer because additional elements are added to it to improve the
performance. However, you have to know how many, how long and where to put
them. That's why we study this stuff.

Wire is cheap after all, and I only > want to crawl up there once.

Crawl up there once and bring a TV antenna with you ... a STORE-BOUGHT TV
antenna. Hang it flat from the rafters. A balun is a little matching
transformer with side-by-side wire connections on one side and a round
coaxial cable connection on the other side. Picture here:

www.summitsource.com/images/products/COTRAN.jpg

Most antennas have two screws for attaching one side of a balun. Connect
your coaxial cable to the other side.

Also, I see instructions that say you should aim the antenna without
defining "aim". Do you align the wire in the direction of the
transmission antenna, or should the wire by perpendicular?

The outline of many TV antennas, viewed from above or below, resembles the
outline of an arrowhead. That's it. The smaller elements are on the end
that's nearer to the TV station. The signal arrives perpendicular to the
alignment of the elements.
http://www.radioshack.com/pwr/content/05/72/2112695_31954_thumbnail.jpg is
an antenna which illustrates the arrowhead concept. The stations are off to
the right side in this picture. I have no idea whether the antenna in the
picture is any good.

If you make a single element antenna, you align it perpendicular with the
arriving signal. These do work pretty well, by the way.
http://www.wfu.edu/~matthews/misc/dipole.html has some step-by-step
instructions for making a folded dipole with ordinary tools.

One last thing: It's not beyond the realm of possibility to make one folded
dipole attic antenna for Channel 6 and a second folded dipole attic antenna
for Channel 9. The Channel 9 antenna just MIGHT also handle 7 and 13 if
you're in a good reception area. You can cable both of them to the TV and
switch between them.

Sal

 

[Rich Webb]

On Tue, 14 Jul 2009 23:28:21 -0700, "Sal M. Onella"
<salmonella@food.poisoning.org> wrote:

[snippety snip some good info from Sal]

Also, to the OP, if you're interested in playing with this some, even
just to the point of seeing what some of the radiation/reception
patterns look like (goes-out signal strength is the same pattern as the
goes-in sensitivity, btw) hop over to http://home.ict.nl/~arivoors/ and
d/l a copy of Arie's version of the 4nec2 antenna modeling software.

There are example files that are similar to typical TV antennas, among
others. You can get a list of channel assignments versus frequency on
Wikipedia.

--
Rich Webb Norfolk, VA

 

[Gnack Nol]

On Tue, 14 Jul 2009 21:55:11 -0700, wdoe999 wrote:

Hi - I need to receive VHF TV (channels 6,7,9,13) and would like to make a
super-duper antenna for inside my attic. I would have thought that I
could easily find (simple) instructions on the internet but can't. Does
anybody have a simple idea that just uses wire (wire should be easy to
attach in an attic).

I've seen some instructions (mostly UHF or DTV) and some of them do
calculations for wavelength (let's say 5 feet). And then, with no
explanation, the guy just says "I made it 10 feet for better reception".
So I ask, can I not then just use the entire length of my attic for
super-duper reception? Wire is cheap after all, and I only want to crawl
up there once.

I don;t have a PhD in antenna making, so a lot of the instructions/ terms
don't mean much to me (dipole, balun, etc). I'm hoping for instructions
such as:
1. Cut a piece of 18gauge coppr wire 5 feet long 2. attach one end to a
rafter.
3. solder the other end to the centre wire of the coax 4 insert tab A into
slot B
etc
etc.


Also, I see instructions that say you should aim the antenna without
defining "aim". Do you allign the wire in the direction of the
transmission antenna, or should the wire by perpendicular?

Thanks

This article gives a great deal of information on HDTV as well as antennas
theory and mounting.
http://www.hdtvprimer.com/

All antennas have definite calculation formulas for
both the element construction and spacing. It varies widely for the type
and frequency range in use. Television antennas generally use a range of
element lengths and are calculated to a specific element spacing simply
stringing random lengths of wire and soldering them to coax is both
wasteful and grossly inefficient.

Here is a simple 4 element design that should be easy to build
http://www.digitalhome.ca/ota/superantenna/index.htm

A normal outdoor antenna can easily be mounted inside the attic. older
UHF antennas will work just as well as those that claim to be HDTV
specific since the operating frequencies are not changed just the
transmission mode.

As for aiming most antennas are directional with the strongest signal being
received from directly in front of the antenna with an angle of 15-30
degrees capture. A few are designed to receive signals from the side but
this mode is usually unreliable.

So you should be sure that you can aim the antenna directly toward the
stations you want for best results and tacking wires on the rafters is
unlikely to give you proper aim enough for good reception.

A copy of the amateur radio antenna gude will give more specific
information if you wish to get a copy.

Gnack

 

[Robert Macy]

On Jul 14, 9:55 pm, wdoe...@yahoo.com wrote:

Hi - I need to receive VHF TV (channels 6,7,9,13) and would like to
make a super-duper antenna for inside my attic.  I would have thought
that I could easily find (simple) instructions on the internet but
can't.  Does anybody have a simple idea that just uses wire (wire
should be easy to attach in an attic).

I've seen some instructions (mostly UHF or DTV) and some of them do
calculations for wavelength (let's say 5 feet).  And then, with no
explanation, the guy just says "I made it 10 feet for better
reception".  So I ask, can I not then just use the entire length of my
attic for super-duper reception?  Wire is cheap after all, and I only
want to crawl up there once.

I don;t have a PhD in antenna making, so a lot of the instructions/
terms don't mean much to me (dipole, balun, etc).  I'm hoping for
instructions such as:
1. Cut a piece of 18gauge coppr wire 5 feet long
2. attach one end to a rafter.
3. solder the other end to the centre wire of the coax
4 insert tab A into slot B
etc
etc.

Also, I see instructions that say you should aim the antenna without
defining "aim".  Do you allign the wire in the direction of the
transmission antenna, or should the wire by perpendicular?

Thanks

Just saw this thread and some very good suggestions.

One thought. Unless your close to the TV Transmitters, stay away from
using wire as the antenna. There is a good reason antennas are made
of tubes, not wire. At high frequency, like television transmission
frequencies, the current creates a repulsion field that pushes the
current away from the center of the conductor. In other words, all
the current travels on the outside surface of the wire. Look up the
term, "skin depth". At frequencies as low as 20MHz, more than 99% of
the current will be within 3 mils of the surface. The only easy way
to lower the losses in the antenna is to use large diameter
conductors, but since the inside of the conductor carries no current,
you don't need metal there, so it is ok to use hollow tubes.

Antenna manufacturers save themselves money by lowering material costs
and shipping weight. They use hollow tubes. If you don't care about
weight or material cost, go ahead and use solid rods, 1/4 inch, or
even 3/8, but stay away from 18 Awg, way too small.

One other thing, nature abhors sharp edges, that's why bubbles are
round, so don't use square tubes or sharp bar stock either. Use
rounded tubes or rods. Even smoothing and polishing the surface
lowers the resistance. When you're done, passivate the surface of the
conductors to prevent corrosion over time. [meaning: paint the
antenna] Over time, corrosion will deteriorate your antenna's
performance. Rounded surfaces also means make your connections smooth
with nice transitions. As in, "if it looks good, it works good." You
can use aluminum if the lengths are continuous and/or you make
connections using constant mechanical pressure, like a "lots of teeth"
star washer that has bitten down through the insulating oxide layer
held with a bolt.

All in all, it seems a lot easier to buy a fringe field antenna and
put that in your attic. But if you do it yourself, hope you're
successful, document what you built, and share it here.

Robert

 

[Roy Lewallen]

I'm sorry, this response contains some misleading advice.

Robert Macy wrote:

Just saw this thread and some very good suggestions.

One thought. Unless your close to the TV Transmitters, stay away from
using wire as the antenna. There is a good reason antennas are made
of tubes, not wire.

The reason usually is physical strength and rigidity. The larger
diameter also increases bandwidth, but often this isn't necessary to
proper operation.

At high frequency, like television transmission
frequencies, the current creates a repulsion field that pushes the
current away from the center of the conductor. In other words, all
the current travels on the outside surface of the wire. Look up the
term, "skin depth". At frequencies as low as 20MHz, more than 99% of
the current will be within 3 mils of the surface. The only easy way
to lower the losses in the antenna is to use large diameter
conductors, but since the inside of the conductor carries no current,
you don't need metal there, so it is ok to use hollow tubes.

This is true. However, in almost all cases the loss caused by using
wire, even very small wire is still negligible. Exceptions are antennas
which are very short in terms of wavelength, particularly at low
frequencies. As frequency increases, the length of an antenna of equal
performance decreases in direct proportion. However, the loss decreases
only as the square root of frequency. So antennas of the same wavelength
size become proportionally less lossy at higher frequencies.

Antenna manufacturers save themselves money by lowering material costs
and shipping weight. They use hollow tubes.

Another important reason for using hollow tubes is structural weight.

If you don't care about
weight or material cost, go ahead and use solid rods, 1/4 inch, or
even 3/8, but stay away from 18 Awg, way too small.

18 AWG wire won't result in appreciable loss for nearly any antenna.

One other thing, nature abhors sharp edges, that's why bubbles are
round, so don't use square tubes or sharp bar stock either.

Square stock is slightly lossier than round, but the loss will be
negligible when unsing any practical size.

Use
rounded tubes or rods. Even smoothing and polishing the surface
lowers the resistance.

Polishing won't make any detectable difference.

When you're done, passivate the surface of the
conductors to prevent corrosion over time. [meaning: paint the
antenna] Over time, corrosion will deteriorate your antenna's
performance.

It depends on the type of corrosion. But it would have to be severe
before becoming so bad as to cause an appreciable reduction in
performance. Aluminum, tin, and some other metals passivate themselves
by forming a hard insulating oxide layer on the outside. Unless you're
in a maritime climate, copper won't deteriorate in a way that matters,
either. Insulated wire is an easy way to prevent corrosion in an
unfavorable climate.

Rounded surfaces also means make your connections smooth
with nice transitions. As in, "if it looks good, it works good."

If only that were true! But unfortunately it isn't.

You
can use aluminum if the lengths are continuous and/or you make
connections using constant mechanical pressure, like a "lots of teeth"
star washer that has bitten down through the insulating oxide layer
held with a bolt.

This can cause more problems than it solves, if the bolt and washer are
the wrong metal such as steel. A good book on Yagi antenna construction
will tell you about techniques for working with aluminum.

All in all, it seems a lot easier to buy a fringe field antenna and
put that in your attic. But if you do it yourself, hope you're
successful, document what you built, and share it here.

With that I agree.

Roy Lewallen, W7EL

 

[RickMerrill]

Roy Lewallen wrote:

I'm sorry, this response contains some misleading advice.

Robert Macy wrote:

Just saw this thread and some very good suggestions.

One thought. Unless your close to the TV Transmitters, stay away from
using wire as the antenna. There is a good reason antennas are made
of tubes, not wire.

The reason usually is physical strength and rigidity. The larger
diameter also increases bandwidth, but often this isn't necessary to
proper operation.

At high frequency, like television transmission
frequencies, the current creates a repulsion field that pushes the
current away from the center of the conductor. In other words, all
the current travels on the outside surface of the wire. Look up the
term, "skin depth". At frequencies as low as 20MHz, more than 99% of
the current will be within 3 mils of the surface. The only easy way
to lower the losses in the antenna is to use large diameter
conductors, but since the inside of the conductor carries no current,
you don't need metal there, so it is ok to use hollow tubes.

This is true. However, in almost all cases the loss caused by using
wire, even very small wire is still negligible. Exceptions are antennas
which are very short in terms of wavelength, particularly at low
frequencies. As frequency increases, the length of an antenna of equal
performance decreases in direct proportion. However, the loss decreases
only as the square root of frequency. So antennas of the same wavelength
size become proportionally less lossy at higher frequencies.

Antenna manufacturers save themselves money by lowering material costs
and shipping weight. They use hollow tubes.

Another important reason for using hollow tubes is structural weight.

If you don't care about
weight or material cost, go ahead and use solid rods, 1/4 inch, or
even 3/8, but stay away from 18 Awg, way too small.

18 AWG wire won't result in appreciable loss for nearly any antenna.

One other thing, nature abhors sharp edges, that's why bubbles are
round, so don't use square tubes or sharp bar stock either.

Square stock is slightly lossier than round, but the loss will be
negligible when unsing any practical size.

Use
rounded tubes or rods. Even smoothing and polishing the surface
lowers the resistance.

Polishing won't make any detectable difference.

When you're done, passivate the surface of the
conductors to prevent corrosion over time. [meaning: paint the
antenna] Over time, corrosion will deteriorate your antenna's
performance.

It depends on the type of corrosion. But it would have to be severe
before becoming so bad as to cause an appreciable reduction in
performance. Aluminum, tin, and some other metals passivate themselves
by forming a hard insulating oxide layer on the outside. Unless you're
in a maritime climate, copper won't deteriorate in a way that matters,
either. Insulated wire is an easy way to prevent corrosion in an
unfavorable climate.

Rounded surfaces also means make your connections smooth
with nice transitions. As in, "if it looks good, it works good."

If only that were true! But unfortunately it isn't.

You
can use aluminum if the lengths are continuous and/or you make
connections using constant mechanical pressure, like a "lots of teeth"
star washer that has bitten down through the insulating oxide layer
held with a bolt.

This can cause more problems than it solves, if the bolt and washer are
the wrong metal such as steel. A good book on Yagi antenna construction
will tell you about techniques for working with aluminum.

All in all, it seems a lot easier to buy a fringe field antenna and
put that in your attic. But if you do it yourself, hope you're
successful, document what you built, and share it here.

With that I agree.

Roy Lewallen, W7EL

Just don't use stranded wire ...

 

[Roy Lewallen]

Correction:

Roy Lewallen wrote:

. . .
This is true. However, in almost all cases the loss caused by using
wire, even very small wire is still negligible. Exceptions are antennas
which are very short in terms of wavelength, particularly at low
frequencies. As frequency increases, the length of an antenna of equal
performance decreases in direct proportion. However, the loss decreases
only as the square root of frequency. So antennas of the same wavelength
size become proportionally less lossy at higher frequencies.
. . .

Loss increases, not decreases, with frequency, in proportion to the
square root of frequency. But the conclusion stated in the last sentence
is correct. If you quadruple the frequency, wires become four times
shorter for the same type of antenna. Assuming you keep the same wire
size, this length change results in one quarter the loss resistance. The
decrease in skin depth due to quadrupling frequency causes an increase
of loss only by a factor of sqrt(4) = 2. The net result is that
quadrupling the frequency cuts the total loss in half.

Roy Lewallen, W7EL

 

[none]

Roy Lewallen wrote:

Correction:

Roy Lewallen wrote:
. . .
This is true. However, in almost all cases the loss caused by using
wire, even very small wire is still negligible. Exceptions are
antennas which are very short in terms of wavelength, particularly at
low frequencies. As frequency increases, the length of an antenna of
equal performance decreases in direct proportion. However, the loss
decreases only as the square root of frequency. So antennas of the
same wavelength size become proportionally less lossy at higher
frequencies.
. . .

Loss increases, not decreases, with frequency, in proportion to the
square root of frequency. But the conclusion stated in the last sentence
is correct. If you quadruple the frequency, wires become four times
shorter for the same type of antenna. Assuming you keep the same wire
size, this length change results in one quarter the loss resistance. The
decrease in skin depth due to quadrupling frequency causes an increase
of loss only by a factor of sqrt(4) = 2. The net result is that
quadrupling the frequency cuts the total loss in half.

Roy Lewallen, W7EL

If we're getting into traditional antenna design details...
I've occasionally wondered why manufacturers still build VHF TV
receiving antennas to match 300 ohm twin-lead wire. They could easily be
made to match 72 ohm or 75 ohm coaxial line. One of these days I think
I'll build a single-channel Quagi TV antenna just to try it out. Pick up
those distant stations...
Those balun transformers have to have SOME loss, don't y'think?

Name and callsign left off 'cause this is a public newsgroup.
Multi-posting removed, 'cause I don't write to folks who can't reply.

 

[Dave Platt]

In article <9rWdnWFDjsgFYsLXnZ2dnUVZ_q6dnZ2d@sysmatrix.net>,
none <""karls\"@(none)"> wrote:

If we're getting into traditional antenna design details...
I've occasionally wondered why manufacturers still build VHF TV
receiving antennas to match 300 ohm twin-lead wire. They could easily be
made to match 72 ohm or 75 ohm coaxial line. One of these days I think
I'll build a single-channel Quagi TV antenna just to try it out. Pick up
those distant stations...
Those balun transformers have to have SOME loss, don't y'think?

I suspect that some of it is nothing more than tradition.

The traditional VHF beam for television is a log-periodic dipole array
(often with a UHF corner reflector added). There are some notes in
the ARRL Antenna Book which suggest that it can be tricky to build an
LPDA with a low feedpoint impedance... the conductors of the
beam-and-transmission-line must be spaced quite closely together. It's
possible to do it, but I suspect that the construction becomes a bit
more critical in dimensioning.

The older-style log periodic design (the one which forms a V in the
vertical plane as well as the horizontal) may not be possible to build
with a low-impedance feedpoint.

It's still necessary (or at least quite desirable) to have a balun of
some sort, to decouple the feedline from the antenna itself. This
will reduce the tendency of the coax shield to act as part of the
antenna... which could cause some amount of ghosting. Some
direct-feed LPDAs run the feedline through the inside of one of the
feeder conductors from the rear to the front, where it comes out and
is attached to both conductors... this serves as a balun. Or, a 1:1
toroidal balun can be used at the feedpoint.

I'm sure you can build a Quagi for direct connection. Ditto for a
Yagi... a WA5VJB half-folded "cheap Yagi" design could be made for a
75-ohm direct connection, or you could use a gamma match on a more
traditional Yagi.

Neither a Yagi nor a Quagi will have the broad frequency response of
an LPDA, though.

--
Dave Platt <dplatt@radagast.org> AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

 

[Sal M. Onella]

<stratus46@yahoo.com> wrote in message
news:538b1dfa-b68d-4d1c-933f-1009c95427c9@m11g2000yqh.googlegroups.com...
On Jul 14, 9:55 pm, wdoe...@yahoo.com wrote:

Hi - I need to receive VHF TV (channels 6,7,9,13) and would like to
make a super-duper antenna for inside my attic.

< snip >

Good grief. Just buy an antenna and open it up in the attic. I've done
it several times and it will cost a lot less in time and aggravation.
Or is your time not worth anything?

Oh, what a shame! Is there no joy in experimenting any more? I have always
felt the essence of our hobby has been somebody saying, "Let's try this and
see what happens." Sometimes it's an enjoyable QSO and sometimes it's a
cloud of acrid smoke. I've had both.

I, too, suggested a store-bought antenna but I went on to suggest other
things to try. The OP seems to have an adventurous spark. I vote for
ENCOURAGE.

73,
Sal
(KD6VKW)

 

On Jul 14, 9:55 pm, wdoe...@yahoo.com wrote:

Hi - I need to receive VHF TV (channels 6,7,9,13) and would like to
make a super-duper antenna for inside my attic.  I would have thought
that I could easily find (simple) instructions on the internet but
can't.  Does anybody have a simple idea that just uses wire (wire
should be easy to attach in an attic).

I've seen some instructions (mostly UHF or DTV) and some of them do
calculations for wavelength (let's say 5 feet).  And then, with no
explanation, the guy just says "I made it 10 feet for better
reception".  So I ask, can I not then just use the entire length of my
attic for super-duper reception?  Wire is cheap after all, and I only
want to crawl up there once.

I don;t have a PhD in antenna making, so a lot of the instructions/
terms don't mean much to me (dipole, balun, etc).  I'm hoping for
instructions such as:
1. Cut a piece of 18gauge coppr wire 5 feet long
2. attach one end to a rafter.
3. solder the other end to the centre wire of the coax
4 insert tab A into slot B
etc
etc.

Also, I see instructions that say you should aim the antenna without
defining "aim".  Do you allign the wire in the direction of the
transmission antenna, or should the wire by perpendicular?

Thanks

Good grief. Just buy an antenna and open it up in the attic. I've done
it several times and it will cost a lot less in time and aggravation.
Or is your time not worth anything?

G˛

 

[Gnack Nol]

some deleted:

If we're getting into traditional antenna design details... I've
occasionally wondered why manufacturers still build VHF TV receiving
antennas to match 300 ohm twin-lead wire. They could easily be made to
match 72 ohm or 75 ohm coaxial line. One of these days I think I'll build
a single-channel Quagi TV antenna just to try it out. Pick up those
distant stations...
Those balun transformers have to have SOME loss, don't y'think?

Name and callsign left off 'cause this is a public newsgroup.
Multi-posting removed, 'cause I don't write to folks who can't reply.

Actually the cost of designing a 75 ohm unbalanced feed antenna would
cause an increase in the overall cost of the antenna because of the
added matching stub which also limits the frequency range of the
antenna, that is why they still build them for 300 ohm balanced feeds.

I just bought a new 8 bay uhf antenna to repace my old rusty one and it
has 300 ohm balanced feed with a 75 ohm balin installed for the down feed.

Yes the standard TV balin is lossy on the order of 2-5 db or more
depending on design. They are seldom designed to high performance
low loss levels used in military and high end commercial units.

Sadly there is no such thing as a TV ranged 300 ohm balanced amp or a TV
with a 300 ohm balanced input any more. 300 ohm ladder line with a twist
every two feet to stop ghosting was far superior and seldom used because
coax was just easier. Ease won out over quality.

Gnack

 

[Richard Clark]

On Fri, 17 Jul 2009 16:16:35 -0700 (PDT), Robert Macy
<macy@california.com> wrote:

We measured huge differences in impedance [measured as improved
Q]

What were the Qs? "Huge differences" is not a quantified, and thus
verifiable claim.

73's
Richard Clark, KB7QHC

 

[Robert Macy]

On Jul 16, 10:58 am, Roy Lewallen <w...@eznec.com> wrote:

I'm sorry, this response contains some misleading advice.

Robert Macy wrote:

Just saw this thread and some very good suggestions.

One thought.  Unless your close to the TV Transmitters, stay away from
using wire as the antenna.  There is a good reason antennas are made
of tubes, not wire.

The reason usually is physical strength and rigidity. The larger
diameter also increases bandwidth, but often this isn't necessary to
proper operation.

 At high frequency, like television transmission
frequencies, the current creates a repulsion field that pushes the
current away from the center of the conductor.  In other words, all
the current travels on the outside surface of the wire.  Look up the
term, "skin depth".  At frequencies as low as 20MHz, more than 99% of
the current will be within 3 mils of the surface.  The only easy way
to lower the losses in the antenna is to use large diameter
conductors, but since the inside of the conductor carries no current,
you don't need metal there, so it is ok to use hollow tubes.

This is true. However, in almost all cases the loss caused by using
wire, even very small wire is still negligible. Exceptions are antennas
which are very short in terms of wavelength, particularly at low
frequencies. As frequency increases, the length of an antenna of equal
performance decreases in direct proportion. However, the loss decreases
only as the square root of frequency. So antennas of the same wavelength
size become proportionally less lossy at higher frequencies.

Antenna manufacturers save themselves money by lowering material costs
and shipping weight.  They use hollow tubes.

Another important reason for using hollow tubes is structural weight.

 If you don't care about
weight or material cost, go ahead and use solid rods, 1/4 inch, or
even 3/8, but stay away from 18 Awg, way too small.

18 AWG wire won't result in appreciable loss for nearly any antenna.

One other thing, nature abhors sharp edges, that's why bubbles are
round, so don't use square tubes or sharp bar stock either.

Square stock is slightly lossier than round, but the loss will be
negligible when unsing any practical size.

 Use
rounded tubes or rods.  Even smoothing and polishing the surface
lowers the resistance.

Polishing won't make any detectable difference.

When you're done, passivate the surface of the
conductors to prevent corrosion over time. [meaning: paint the
antenna]  Over time, corrosion will deteriorate your antenna's
performance.

It depends on the type of corrosion. But it would have to be severe
before becoming so bad as to cause an appreciable reduction in
performance. Aluminum, tin, and some other metals passivate themselves
by forming a hard insulating oxide layer on the outside. Unless you're
in a maritime climate, copper won't deteriorate in a way that matters,
either. Insulated wire is an easy way to prevent corrosion in an
unfavorable climate.

Rounded surfaces also means make your connections smooth
with nice transitions.  As in, "if it looks good, it works good."

If only that were true! But unfortunately it isn't.

You
can use aluminum if the lengths are continuous and/or you make
connections using constant mechanical pressure, like a "lots of teeth"
star washer that has bitten down through the insulating oxide layer
held with a bolt.

This can cause more problems than it solves, if the bolt and washer are
the wrong metal such as steel. A good book on Yagi antenna construction
will tell you about techniques for working with aluminum.

All in all, it seems a lot easier to buy a fringe field antenna and
put that in your attic.  But if you do it yourself, hope you're
successful, document what you built, and share it here.

With that I agree.

Roy Lewallen, W7EL

My advice was NOT misleading. Yours was a very poor choice of word.
Misleading means the suggested effort would result in moving away from
an optimum solution. ALL of my advice leads to better solutions and
is therefore "not misleading" and I stand behind my suggestions.

You are very correct on catching the lack of thoroughness addressing
'mixed metal' contacts. Yes, a lot of electrolytic action happens at
the junction of dissimilar materials. One must be extremely careful
when making such contacts.

Had you criticized my comments by suggesting that many of the efforts
involved will not yield noticeable improvement [especially to a
novice], I would have accepted that. After working for years in low
noise, high performance systems these techniques have become de
rigueur for initial construction. The OP probably would not notice
improvements except in the most fringe of conditions.

Yes, the diameter of the rod will broaden bandwidth, but elements
having ratios on the order of 80:1, the effect on bandwidth won't be
very noticeable. Length variations and spacing will have more
impact.

Regarding skin depth of a conductor: Always keep in mind that the
skin depth equation is based upon the assumption of PLANAR wave. The
equation is extremely simple and easily memorized as the square root
of 2 divided by three terms:

skin depth(in meters) = sqrt( 2/(p*o*w) )
where p = magnetic permeability
o = conductivity
w = frequency in radians

for copper, p = 4 pi 10-7
o = 58 MS/m
w = 2*pi*f, with f in Hz

results are in meters, so I suggest using an Excel spreadsheet
formula.

skin depth of copper at 80MHz is 0.3 mils! 99% of the current is in
less than 1 mil of the conductor.

Using finite element analyses [femm 4.2] techniques it is easy to
calculate the impedance of a conductor as long as the dimensions stay
below 1/10 of wavelength. For 80MHz that would be 1.2 feet. At
80MHz, 18 Awg copper wire is approx 220 milliohms per foot and 3/8
inch aluminum tubing is approx 36 milliohms per foot. Neither of
these impedances would have much impact to the signal coming from the
377 ohm source impedance of free space.

Normally we would have predicted the decrease in impedance by applying
the ratio of the increased perimeter reduced by the less conductive
aluminum. The ratio of perimeter is 0.375/.04 or approx 9.4, but
aluminum is not as conductive as copper so the conductivity ratio is
25/58 for a total change in resistance by approx 0.375/.04*(25/58) 4.04 improvement. Finite element analyses calculates the improvement
to be more than 50% higher than that. [It's caused by the small
radius of the wire.] Plotting the current density down into the
conductors shows what happened. The 'effective' skin depth in the 18
Awg wire is about 50% less than in the aluminum, all due to the
reduced radius of the outside of the conductor. As I said, nature
hates sharp edges.

Again, as the element 'taps' into free space the 377 ohms of free
space predicts a difference of less than 0.005 dB on the signal. So
there will indeed be an extremely small effect from using 18 Awg wire
or 3/8 tubing on the received signal strength.

If the antenna were to be much smaller than wavelengths and
capacitance were added to resonate the elements, THEN the impedance
dfference would become noticeable and affect how much signal is
available to the receiver.

Regarding bar stock? picture the current concentrating at each of the
four corner edges, with the current not being uniformly distributed
around the perimeter. That would almost be equivalent to having 4
parallel small diameter wires mounted on 3/8 inch centers! Really
wasted the metal.

Regarding the importance of smoothness: My outside antenna became
badly pitted from atmosphere, even with aluminum developing an oxide
layer, it still corroded. The roughness lowered the gain of the
antenna enough to notice it on the reception from fringe stations.
Years ago [and at higher frequencies] we plated metal in our
resonators with silver *and* polished the silver to get the impedance
down. We measured huge differences in impedance [measured as improved
Q] as we polished the surface down to mirror finishes.

Whoever suggested the ARRL Antenna handbook is right. Great book.

Regards,
Robert

 

[Alan Douglas]

Hi - I need to receive VHF TV (channels 6,7,9,13) and would like to
make a super-duper antenna for inside my attic. I would have thought
that I could easily find (simple) instructions on the internet but
can't. Does anybody have a simple idea that just uses wire (wire
should be easy to attach in an attic).

There was an article in Electronics World in December 1967 by Harold
Pruett titled "Designs for Log-Periodic FM & TV antennas". He used two
lengths of hookup wire, attached to a wooden frame in a zigzag
pattern, and gives all the dimensions needed. I built one then and it
has worked fine ever since, though now there's nothing to receive in
this area so I've switched to a UHF-only antenna in the attic. I can
mail you xeroxes of the article. I'm adouglas (at) gis.net.

Alan

 

[Richard Clark]

On Sat, 18 Jul 2009 08:31:36 -0700 (PDT), Robert Macy
<macy@california.com> wrote:

We measured huge differences in impedance [measured as improved
Q]

What were the Qs?  "Huge differences" is not a quantified, and thus
verifiable claim.


Memory serves Q went from 400/600 range up to the 8000 range.

For what band?

73's
Richard Clark, KB7QHC

 

[Robert Macy]

On Jul 17, 4:45 pm, Richard Clark <kb7...@comcast.net> wrote:

On Fri, 17 Jul 2009 16:16:35 -0700 (PDT), Robert Macy

m...@california.com> wrote:
We measured huge differences in impedance [measured as improved
Q]

What were the Qs?  "Huge differences" is not a quantified, and thus
verifiable claim.

73's
Richard Clark, KB7QHC

Memory serves Q went from 400/600 range up to the 8000 range.