Impedance mismatch graded line

[Doug Goncz]

Hello All.

I have a wonderful seven element antenna, highly directional, that is capable
of capturing WHFS from Seven Corners, VA. However, the natural impedance of the
antenna is 300 ohms, that is, it is best suited for use at the intended
frequency range of 88-108 MHz with the classic dual lead transmission line,
flat, about 3/8 inch wide, which, if extended infinitely, would present a load
of 300 ohms to a source in that range.

For effective operation, the input to the receiver must have the same
impedance.

Mine doesn't.

How do I know?

It's obvious.

The antenna that came with my Wizard Radio ISA card is an open dipole made of
about 22 gage wire that terminates in a 1/8 inch 3 contact phone plug!

This I figure has a characteristic impedance of like twinax, about 100 ohms.

So I soldered a phone plug onto the 300 ohm antenna wire and got lousy
reception of WHFS.

And I sat and thought for a few days, and I had an idea.

I removed the plug, separated the wires for about a foot by slicing down the
middle, and shaved each one down until it was a round lead. Then I laced them
loosely together with dental floss, making a tapered air space dual lead
transmission line. And I resoldered the ends taking greater care, not using the
termination holes in the tabs on the phone jack, but rather scraping the tabs,
fluxing and tinning them, fluxing and tinning the leads, and sweating them in
place neatly.

The result was an improvement, but there are two problems.

I am only receiving mono, indicating low signal strength. And people in the
room can affect reception by sitting, standing, and moving. And I pick up
interference of all kinds, since the line isn't shielded.

I have a call to L-com for twenty feet of 78 ohm twinax. I intend to use all
three connectors this time, tip and ring for the signal, and barrel for the
sheild. I intend to run the shielded twinax to the tip of the antenna and make
a mechanical connection, with a plastic spacer and some tape.

Then comes the fun. At this point I need to cut and remove the braid and
install spade lugs on the twinax leads so they are air spaced at the antenna
lugs the given spacing, about four inches, add #6-32 hex supports, and fasten
the lugs up above the boom, about 1/4 inch.

So the result will be a boom ground plane (sort of), and two tensioned leads
running from two inch spacing to adjacent at the twinax.

We'll just see if it works, and I shall report here.

You see, the varying air spacing between the leads acts as an impedance
matcher, eliminating much of the reflection that I am having now. I can't
measure it, I just know it is there.

It is like a capacitative transformer.

You're probably going to tell me the spacing should be parabolic, hyperbolic,
or exponential. C'mon.

I'm talking about stressing the leads to make the adapter. What could I do,
insert a piece of foam laminate board cut to shape with grooves routed in the
edges? Well, actually, I could, so if there is a shape other than a triangle
that I can use, I'd appreciate your opinion, as I just love WHFS!


Yours,

Doug Goncz ( ftp://users.aol.com/DGoncz/ )

Read about my physics project at NVCC:
http://groups.google.com/groups?q=dgoncz&scoring=d plus
"bicycle", "fluorescent", "inverter", "flywheel", "ultracapacitor", etc.
in the search box

 

[Henry Kolesnik]

try this group:
rec.radio.amateur.antenna

" Doug Goncz " <dgoncz@aol.com.bat.exe> wrote in message
news:20040519152849.26642.00001702@mb-m11.aol.com...

Hello All.

I have a wonderful seven element antenna, highly directional, that is
capable
of capturing WHFS from Seven Corners, VA. However, the natural impedance
of the
antenna is 300 ohms, that is, it is best suited for use at the intended
frequency range of 88-108 MHz with the classic dual lead transmission
line,
flat, about 3/8 inch wide, which, if extended infinitely, would present a
load
of 300 ohms to a source in that range.

For effective operation, the input to the receiver must have the same
impedance.

Mine doesn't.

How do I know?

It's obvious.

The antenna that came with my Wizard Radio ISA card is an open dipole made
of
about 22 gage wire that terminates in a 1/8 inch 3 contact phone plug!

This I figure has a characteristic impedance of like twinax, about 100
ohms.

So I soldered a phone plug onto the 300 ohm antenna wire and got lousy
reception of WHFS.

And I sat and thought for a few days, and I had an idea.

I removed the plug, separated the wires for about a foot by slicing down
the
middle, and shaved each one down until it was a round lead. Then I laced
them
loosely together with dental floss, making a tapered air space dual lead
transmission line. And I resoldered the ends taking greater care, not
using the
termination holes in the tabs on the phone jack, but rather scraping the
tabs,
fluxing and tinning them, fluxing and tinning the leads, and sweating them
in
place neatly.

The result was an improvement, but there are two problems.

I am only receiving mono, indicating low signal strength. And people in
the
room can affect reception by sitting, standing, and moving. And I pick up
interference of all kinds, since the line isn't shielded.

I have a call to L-com for twenty feet of 78 ohm twinax. I intend to use
all
three connectors this time, tip and ring for the signal, and barrel for
the
sheild. I intend to run the shielded twinax to the tip of the antenna and
make
a mechanical connection, with a plastic spacer and some tape.

Then comes the fun. At this point I need to cut and remove the braid and
install spade lugs on the twinax leads so they are air spaced at the
antenna
lugs the given spacing, about four inches, add #6-32 hex supports, and
fasten
the lugs up above the boom, about 1/4 inch.

So the result will be a boom ground plane (sort of), and two tensioned
leads
running from two inch spacing to adjacent at the twinax.

We'll just see if it works, and I shall report here.

You see, the varying air spacing between the leads acts as an impedance
matcher, eliminating much of the reflection that I am having now. I can't
measure it, I just know it is there.

It is like a capacitative transformer.

You're probably going to tell me the spacing should be parabolic,
hyperbolic,
or exponential. C'mon.

I'm talking about stressing the leads to make the adapter. What could I
do,
insert a piece of foam laminate board cut to shape with grooves routed in
the
edges? Well, actually, I could, so if there is a shape other than a
triangle
that I can use, I'd appreciate your opinion, as I just love WHFS!


Yours,

Doug Goncz ( ftp://users.aol.com/DGoncz/ )

Read about my physics project at NVCC:
http://groups.google.com/groups?q=dgoncz&scoring=d plus
"bicycle", "fluorescent", "inverter", "flywheel", "ultracapacitor", etc.
in the search box

 

[Jack Smith]

On 19 May 2004 19:28:49 GMT, dgoncz@aol.com.bat.exe ( Doug Goncz )
wrote:

Hello All.

I have a wonderful seven element antenna, highly directional, that is capable
of capturing WHFS from Seven Corners, VA. However, the natural impedance of the
antenna is 300 ohms, that is, it is best suited for use at the intended
frequency range of 88-108 MHz with the classic dual lead transmission line,
flat, about 3/8 inch wide, which, if extended infinitely, would present a load
of 300 ohms to a source in that range.

For effective operation, the input to the receiver must have the same
impedance.

Mine doesn't.

How do I know?

It's obvious.

The antenna that came with my Wizard Radio ISA card is an open dipole made of
about 22 gage wire that terminates in a 1/8 inch 3 contact phone plug!

This I figure has a characteristic impedance of like twinax, about 100 ohms.

So I soldered a phone plug onto the 300 ohm antenna wire and got lousy
reception of WHFS.

And I sat and thought for a few days, and I had an idea.

I removed the plug, separated the wires for about a foot by slicing down the
middle, and shaved each one down until it was a round lead. Then I laced them
loosely together with dental floss, making a tapered air space dual lead
transmission line. And I resoldered the ends taking greater care, not using the
termination holes in the tabs on the phone jack, but rather scraping the tabs,
fluxing and tinning them, fluxing and tinning the leads, and sweating them in
place neatly.

The result was an improvement, but there are two problems.

I am only receiving mono, indicating low signal strength. And people in the
room can affect reception by sitting, standing, and moving. And I pick up
interference of all kinds, since the line isn't shielded.

I have a call to L-com for twenty feet of 78 ohm twinax. I intend to use all
three connectors this time, tip and ring for the signal, and barrel for the
sheild. I intend to run the shielded twinax to the tip of the antenna and make
a mechanical connection, with a plastic spacer and some tape.

Then comes the fun. At this point I need to cut and remove the braid and
install spade lugs on the twinax leads so they are air spaced at the antenna
lugs the given spacing, about four inches, add #6-32 hex supports, and fasten
the lugs up above the boom, about 1/4 inch.

So the result will be a boom ground plane (sort of), and two tensioned leads
running from two inch spacing to adjacent at the twinax.

We'll just see if it works, and I shall report here.

You see, the varying air spacing between the leads acts as an impedance
matcher, eliminating much of the reflection that I am having now. I can't
measure it, I just know it is there.

It is like a capacitative transformer.

You're probably going to tell me the spacing should be parabolic, hyperbolic,
or exponential. C'mon.

I'm talking about stressing the leads to make the adapter. What could I do,
insert a piece of foam laminate board cut to shape with grooves routed in the
edges? Well, actually, I could, so if there is a shape other than a triangle
that I can use, I'd appreciate your opinion, as I just love WHFS!


Yours,

Doug Goncz ( ftp://users.aol.com/DGoncz/ )

Read about my physics project at NVCC:
http://groups.google.com/groups?q=dgoncz&scoring=d plus
"bicycle", "fluorescent", "inverter", "flywheel", "ultracapacitor", etc.
in the search box

Actually, the impedance of a half-wave dipole is closer to 72 ohms
than 100.

If the input is balanced (tip and sleeve both above ground), then the
input impedance is likely 300 ohms. If the input is unbalanced, then I
would bet on 75 ohm input impedance. It's even possible the input has
both a 300 ohm and a 75 ohm input option.

The first experiment I would try is with an inexpensive 300:75 balun
available at Radio Shack or the equivalent.


Jack Smith

 

[Roy McCammon]

Doug Goncz wrote:

Hello All.

I have a wonderful seven element antenna, highly directional, that is capable
of capturing WHFS from Seven Corners, VA. However, the natural impedance of the
antenna is 300 ohms, that is, it is best suited for use at the intended
frequency range of 88-108 MHz with the classic dual lead transmission line,
flat, about 3/8 inch wide, which, if extended infinitely, would present a load
of 300 ohms to a source in that range.

My first thought is that commercial broadcast reception
is most often limited by s/n ratio, which, if the antenna
treats the signal and noise equally ought not to be
effected by the mismatch. But if you are right at
the point of insufficient signal strength, it might
make a difference as well as the positions of people
and things.

Maybe the directivity of the antenna is messed up
by the mismatch? If so maybe a resistive pad
would help.

But ultimately I imagine the best improvement
would be increased elevation of the antenna.




--
local optimization seldom leads to global optimization

my e-mail address is: <my first name> <my last name> AT mmm DOT com

 

[Tam/WB2TT]

" Doug Goncz " <dgoncz@aol.com.bat.exe> wrote in message
news:20040519152849.26642.00001702@mb-m11.aol.com...

Hello All.

I have a wonderful seven element antenna, highly directional, that is
capable
of capturing WHFS from Seven Corners, VA. However, the natural impedance
of the
antenna is 300 ohms, that is, it is best suited for use at the intended
frequency range of 88-108 MHz with the classic dual lead transmission
line,
flat, about 3/8 inch wide, which, if extended infinitely, would present a
load
of 300 ohms to a source in that range.

For effective operation, the input to the receiver must have the same
impedance.

Mine doesn't.

How do I know?

It's obvious.

The antenna that came with my Wizard Radio ISA card is an open dipole made
of
about 22 gage wire that terminates in a 1/8 inch 3 contact phone plug!

This I figure has a characteristic impedance of like twinax, about 100
ohms.

So I soldered a phone plug onto the 300 ohm antenna wire and got lousy
reception of WHFS.

And I sat and thought for a few days, and I had an idea.

I removed the plug, separated the wires for about a foot by slicing down
the
middle, and shaved each one down until it was a round lead. Then I laced
them
loosely together with dental floss, making a tapered air space dual lead
transmission line. And I resoldered the ends taking greater care, not
using the
termination holes in the tabs on the phone jack, but rather scraping the
tabs,
fluxing and tinning them, fluxing and tinning the leads, and sweating them
in
place neatly.

The result was an improvement, but there are two problems.

I am only receiving mono, indicating low signal strength. And people in
the
room can affect reception by sitting, standing, and moving. And I pick up
interference of all kinds, since the line isn't shielded.

I have a call to L-com for twenty feet of 78 ohm twinax. I intend to use
all
three connectors this time, tip and ring for the signal, and barrel for
the
sheild. I intend to run the shielded twinax to the tip of the antenna and
make
a mechanical connection, with a plastic spacer and some tape.

Then comes the fun. At this point I need to cut and remove the braid and
install spade lugs on the twinax leads so they are air spaced at the
antenna
lugs the given spacing, about four inches, add #6-32 hex supports, and
fasten
the lugs up above the boom, about 1/4 inch.

So the result will be a boom ground plane (sort of), and two tensioned
leads
running from two inch spacing to adjacent at the twinax.

We'll just see if it works, and I shall report here.

You see, the varying air spacing between the leads acts as an impedance
matcher, eliminating much of the reflection that I am having now. I can't
measure it, I just know it is there.

It is like a capacitative transformer.

You're probably going to tell me the spacing should be parabolic,
hyperbolic,
or exponential. C'mon.

I'm talking about stressing the leads to make the adapter. What could I
do,
insert a piece of foam laminate board cut to shape with grooves routed in
the
edges? Well, actually, I could, so if there is a shape other than a
triangle
that I can use, I'd appreciate your opinion, as I just love WHFS!


Yours,

Doug Goncz ( ftp://users.aol.com/DGoncz/ )

Read about my physics project at NVCC:
http://groups.google.com/groups?q=dgoncz&scoring=d plus
"bicycle", "fluorescent", "inverter", "flywheel", "ultracapacitor", etc.
in the search box

Try the 4:1 balun first. Look at the stuff that came with your TV or VCR.
You probably got one.

Tam

 

[Tom Bruhns]

Sounds to me like you are kind of trying to use a sledge hammer to
drive a brad. I'd recommend using a 300-75 balun at the antenna, and
a mast mounted preamp if the signal strength is still too low, and
standard 75 ohm coax down to the receiver.

By the way, receivers seldom have the input impedance of the line
they're designed to connect to. The reason for this is that the best
noise figure is genarally NOT obtained when the input is driven from a
matched source impedance.

It's also true that the antenna may not really be presenting a 300 ohm
source impedance. If it's a standard "folded dipole" driven element
with a bunch of parasitic elements, the impedance is generally lowered
by the parasitic elements. But don't get too hung up on all that.
There is benefit to using a balun where appropriate, and there may be
an advantage to using a good mast-mounted preamp to avoid the 1-for-1
noise figure degradation of the feedline's loss, but moderate
impedance mismatches should not be hurting you too much.

Sorry, on re-reading your message it appears you were not asking for
advice, only reporting what you were going to do. So please disregard
the above advice. ;-) For your tapered line, just use a couple
meters of a 12AWG copper wire pair, tapered as you wish, spaced
perhaps with little pieces cut from a plastic (polyethylene) milk jug.
They should, of course, be quite close together at the "78 ohm" end.
I'm sure you have the formula for parallel-wire-line impedance.

Cheers,
Tom

dgoncz@aol.com.bat.exe ( Doug Goncz ) wrote in message news:<20040519152849.26642.00001702@mb-m11.aol.com>...

Hello All.

I have a wonderful seven element antenna, highly directional, that is capable
of capturing WHFS from Seven Corners, VA. However, the natural impedance of the
antenna is 300 ohms, that is, it is best suited for use at the intended
frequency range of 88-108 MHz with the classic dual lead transmission line,
flat, about 3/8 inch wide, which, if extended infinitely, would present a load
of 300 ohms to a source in that range.

For effective operation, the input to the receiver must have the same
impedance.

Mine doesn't.

How do I know?

It's obvious.

The antenna that came with my Wizard Radio ISA card is an open dipole made of
about 22 gage wire that terminates in a 1/8 inch 3 contact phone plug!

This I figure has a characteristic impedance of like twinax, about 100 ohms.

So I soldered a phone plug onto the 300 ohm antenna wire and got lousy
reception of WHFS.

And I sat and thought for a few days, and I had an idea.

I removed the plug, separated the wires for about a foot by slicing down the
middle, and shaved each one down until it was a round lead. Then I laced them
loosely together with dental floss, making a tapered air space dual lead
transmission line. And I resoldered the ends taking greater care, not using the
termination holes in the tabs on the phone jack, but rather scraping the tabs,
fluxing and tinning them, fluxing and tinning the leads, and sweating them in
place neatly.

The result was an improvement, but there are two problems.

I am only receiving mono, indicating low signal strength. And people in the
room can affect reception by sitting, standing, and moving. And I pick up
interference of all kinds, since the line isn't shielded.

I have a call to L-com for twenty feet of 78 ohm twinax. I intend to use all
three connectors this time, tip and ring for the signal, and barrel for the
sheild. I intend to run the shielded twinax to the tip of the antenna and make
a mechanical connection, with a plastic spacer and some tape.

Then comes the fun. At this point I need to cut and remove the braid and
install spade lugs on the twinax leads so they are air spaced at the antenna
lugs the given spacing, about four inches, add #6-32 hex supports, and fasten
the lugs up above the boom, about 1/4 inch.

So the result will be a boom ground plane (sort of), and two tensioned leads
running from two inch spacing to adjacent at the twinax.

We'll just see if it works, and I shall report here.

You see, the varying air spacing between the leads acts as an impedance
matcher, eliminating much of the reflection that I am having now. I can't
measure it, I just know it is there.

It is like a capacitative transformer.

You're probably going to tell me the spacing should be parabolic, hyperbolic,
or exponential. C'mon.

I'm talking about stressing the leads to make the adapter. What could I do,
insert a piece of foam laminate board cut to shape with grooves routed in the
edges? Well, actually, I could, so if there is a shape other than a triangle
that I can use, I'd appreciate your opinion, as I just love WHFS!


Yours,

Doug Goncz ( ftp://users.aol.com/DGoncz/ )

Read about my physics project at NVCC:
http://groups.google.com/groups?q=dgoncz&scoring=d plus
"bicycle", "fluorescent", "inverter", "flywheel", "ultracapacitor", etc.
in the search box

 

[Wayne Shanks]

WHFS SUCKS! ... Now.



Doug Goncz wrote:

Hello All.

I have a wonderful seven element antenna, highly directional, that is capable
of capturing WHFS from Seven Corners, VA. However, the natural impedance of the
antenna is 300 ohms, that is, it is best suited for use at the intended
frequency range of 88-108 MHz with the classic dual lead transmission line,
flat, about 3/8 inch wide, which, if extended infinitely, would present a load
of 300 ohms to a source in that range.

For effective operation, the input to the receiver must have the same
impedance.

Mine doesn't.

How do I know?

It's obvious.

The antenna that came with my Wizard Radio ISA card is an open dipole made of
about 22 gage wire that terminates in a 1/8 inch 3 contact phone plug!

This I figure has a characteristic impedance of like twinax, about 100 ohms.

So I soldered a phone plug onto the 300 ohm antenna wire and got lousy
reception of WHFS.

And I sat and thought for a few days, and I had an idea.

I removed the plug, separated the wires for about a foot by slicing down the
middle, and shaved each one down until it was a round lead. Then I laced them
loosely together with dental floss, making a tapered air space dual lead
transmission line. And I resoldered the ends taking greater care, not using the
termination holes in the tabs on the phone jack, but rather scraping the tabs,
fluxing and tinning them, fluxing and tinning the leads, and sweating them in
place neatly.

The result was an improvement, but there are two problems.

I am only receiving mono, indicating low signal strength. And people in the
room can affect reception by sitting, standing, and moving. And I pick up
interference of all kinds, since the line isn't shielded.

I have a call to L-com for twenty feet of 78 ohm twinax. I intend to use all
three connectors this time, tip and ring for the signal, and barrel for the
sheild. I intend to run the shielded twinax to the tip of the antenna and make
a mechanical connection, with a plastic spacer and some tape.

Then comes the fun. At this point I need to cut and remove the braid and
install spade lugs on the twinax leads so they are air spaced at the antenna
lugs the given spacing, about four inches, add #6-32 hex supports, and fasten
the lugs up above the boom, about 1/4 inch.

So the result will be a boom ground plane (sort of), and two tensioned leads
running from two inch spacing to adjacent at the twinax.

We'll just see if it works, and I shall report here.

You see, the varying air spacing between the leads acts as an impedance
matcher, eliminating much of the reflection that I am having now. I can't
measure it, I just know it is there.

It is like a capacitative transformer.

You're probably going to tell me the spacing should be parabolic, hyperbolic,
or exponential. C'mon.

I'm talking about stressing the leads to make the adapter. What could I do,
insert a piece of foam laminate board cut to shape with grooves routed in the
edges? Well, actually, I could, so if there is a shape other than a triangle
that I can use, I'd appreciate your opinion, as I just love WHFS!


Yours,

Doug Goncz ( ftp://users.aol.com/DGoncz/ )

Read about my physics project at NVCC:
http://groups.google.com/groups?q=dgoncz&scoring=d plus
"bicycle", "fluorescent", "inverter", "flywheel", "ultracapacitor", etc.
in the search box

--
"Anyone over 27 years of age that's not married is a menace to society"
Brigham Young

"Dam Right"
Wayne Shanks

 

[Robert Baer]

Wayne Shanks wrote:

WHFS SUCKS! ... Now.

Doug Goncz wrote:
Hello All.

I have a wonderful seven element antenna, highly directional, that is capable
of capturing WHFS from Seven Corners, VA. However, the natural impedance of the
antenna is 300 ohms, that is, it is best suited for use at the intended
frequency range of 88-108 MHz with the classic dual lead transmission line,
flat, about 3/8 inch wide, which, if extended infinitely, would present a load
of 300 ohms to a source in that range.

For effective operation, the input to the receiver must have the same
impedance.

Mine doesn't.

How do I know?

It's obvious.

The antenna that came with my Wizard Radio ISA card is an open dipole made of
about 22 gage wire that terminates in a 1/8 inch 3 contact phone plug!

This I figure has a characteristic impedance of like twinax, about 100 ohms.

So I soldered a phone plug onto the 300 ohm antenna wire and got lousy
reception of WHFS.

And I sat and thought for a few days, and I had an idea.

I removed the plug, separated the wires for about a foot by slicing down the
middle, and shaved each one down until it was a round lead. Then I laced them
loosely together with dental floss, making a tapered air space dual lead
transmission line. And I resoldered the ends taking greater care, not using the
termination holes in the tabs on the phone jack, but rather scraping the tabs,
fluxing and tinning them, fluxing and tinning the leads, and sweating them in
place neatly.

The result was an improvement, but there are two problems.

I am only receiving mono, indicating low signal strength. And people in the
room can affect reception by sitting, standing, and moving. And I pick up
interference of all kinds, since the line isn't shielded.

I have a call to L-com for twenty feet of 78 ohm twinax. I intend to use all
three connectors this time, tip and ring for the signal, and barrel for the
sheild. I intend to run the shielded twinax to the tip of the antenna and make
a mechanical connection, with a plastic spacer and some tape.

Then comes the fun. At this point I need to cut and remove the braid and
install spade lugs on the twinax leads so they are air spaced at the antenna
lugs the given spacing, about four inches, add #6-32 hex supports, and fasten
the lugs up above the boom, about 1/4 inch.

So the result will be a boom ground plane (sort of), and two tensioned leads
running from two inch spacing to adjacent at the twinax.

We'll just see if it works, and I shall report here.

You see, the varying air spacing between the leads acts as an impedance
matcher, eliminating much of the reflection that I am having now. I can't
measure it, I just know it is there.

It is like a capacitative transformer.

You're probably going to tell me the spacing should be parabolic, hyperbolic,
or exponential. C'mon.

I'm talking about stressing the leads to make the adapter. What could I do,
insert a piece of foam laminate board cut to shape with grooves routed in the
edges? Well, actually, I could, so if there is a shape other than a triangle
that I can use, I'd appreciate your opinion, as I just love WHFS!


Yours,

Doug Goncz ( ftp://users.aol.com/DGoncz/ )

Read about my physics project at NVCC:
http://groups.google.com/groups?q=dgoncz&scoring=d plus
"bicycle", "fluorescent", "inverter", "flywheel", "ultracapacitor", etc.
in the search box

--
"Anyone over 27 years of age that's not married is a menace to society"
Brigham Young

"Dam Right"
Wayne Shanks

Perhaps the use of a 300 ohm to 75 ohm bakun would be the solution...

 

[Doug Goncz]

From: Jack Smith Jack.Smith@cox.net

Actually, the impedance of a half-wave dipole is closer to 72 ohms
than 100.

This makes sense. The impedance of twinax is about 78 ohms. The input to the
radio is just like two parallel twinax, I am not good at estimating the gage, I
am looking at a picture. Say 20 gage. Twinax comes twisted. And it's definitely
polyethylene, I work with plastics.

If the input is balanced (tip and sleeve both above ground), then the
input impedance is likely 300 ohms.

Well, yes, but the wire coming to the plug is definitely not 300. The barrel is
grounded at the chassis, tip and sleeve go to the two legs of the dipole.

It's even possible the input has
both a 300 ohm and a 75 ohm input option.

I've examined the board and found no jumper but a memory jumper.

The first experiment I would try is with an inexpensive 300:75 balun
available at Radio Shack or the equivalent.

I am sorry I didn't mention I tried that. That's what got me started. I
connected a channel 3-13 American TV antenna with an unbalanced presumably 75
ohm output, an F connector, to a 75/300 balun and stripped the ends of the 300
ohm output. I tinned them, and bent the tips back. I forced the vees formed
into two RCA jacks wired to an 1/8 inch three conductor phone plug. This was a
vast improvement over the included dipole antenna.


Yours,

Doug Goncz ( ftp://users.aol.com/DGoncz/ )

Read about my physics project at NVCC:
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[Doug Goncz]

From: Roy McCammon rbmccammon@mmm.com

My first thought is that commercial broadcast reception
is most often limited by s/n ratio, which, if the antenna
treats the signal and noise equally ought not to be
effected by the mismatch.

It probably does but I they play rock and roll, highly compressed and limited.
The noise isn't audible unless you really crank it up and these are little
speakers currently.

But if you are right at
the point of insufficient signal strength, it might
make a difference as well as the positions of people
and things.

Yes, with low signal, noise heard comes from within the detector, I think is
where it comes from. I am having less trouble with positions of people and
things now.

Maybe the directivity of the antenna is messed up
by the mismatch?

That can happen, I think.

If so maybe a resistive pad
would help.

How would I do that? I couldn't find a schematic with Google. I am not familiar
with the term but can assemble one from a description. I've seen the inside of
a 75 ohm splitter and underand how that works.

But ultimately I imagine the best improvement
would be increased elevation of the antenna.

Alas, third floor ceiling is as high as the condominium association will let it
go.


Yours,

Doug Goncz ( ftp://users.aol.com/DGoncz/ )

Read about my physics project at NVCC:
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[Doug Goncz]

From: "Tam/WB2TT" t-tammaru@c0mca$t.net

Try the 4:1 balun first. Look at the stuff that came with your TV or VCR.
You probably got one.

Tam

Yes, I did try one. See my answer to Jack.


Yours,

Doug Goncz ( ftp://users.aol.com/DGoncz/ )

Read about my physics project at NVCC:
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[Doug Goncz]

By the way, receivers seldom have the input impedance of the line
they're designed to connect to. The reason for this is that the best
noise figure is genarally NOT obtained when the input is driven from a
matched source impedance.

Wow. I never considered that. I just thought the higher the signal coming was
the better the quieting would be. I'd like to learn why this is so.

But still, the line going into the receiver looks like it's 78 ohms balanced,
so it makes sense to try to maintain that, right? Regardless of what the actual
input impedance of the receiver is, yes?

It's also true that the antenna may not really be presenting a 300 ohm
source impedance.

OK.

If it's a standard "folded dipole" driven element
with a bunch of parasitic elements, the impedance is generally lowered
by the parasitic elements.

These are open dipoles with what might be parasitic elements. The open elements
are cross wired 1,2,3.

I put the hanger bolt over the cross nearest the balance point. I dressed the
cross wires to the center of the bolt (a U-bolt). I tinned the antenna lead
ends and removed extra plastic. I bent them into hooks.

So now the antenna is at least hanging correctly with the feed on the bottom.
Now the line drapes away from the antenna. A good bit better. Getting stereo
now mid day.

Sorry, on re-reading your message it appears you were not asking for
advice, only reporting what you were going to do. So please disregard
the above advice. ;-)

OK, I won't get hung up on twinax.

I've dressed the end of a spare piece of line that is long enough to be
installed. I split it down the middle as before but didn't strip the lines. I
left the flaps attached. I laced it with fine thread forming a smooth curve as
the lines got closer together. It bends about 60 degrees just from the lacing.
I glued the thread in place and it's drying. So now it's spaced with
polyethylene and glue. I hope the glue doesn't throw it off.

At the end, which hasn't been soldered yet, the flaps are parallel and the
wires adjacent. Since I have searched and even asked my professor for twinax,
this line will be the last try. It should do fine.

I don't know if I'll do the spacing along the boom, now with the 300 ohm line.
That seems overly subtle. With rock, you don't need a great S/N ratio, so I'll
settle as soon as I try this last tweak.

For your tapered line, just use a couple
meters of a 12AWG copper wire pair, tapered as you wish, spaced
perhaps with little pieces cut from a plastic (polyethylene) milk jug.
They should, of course, be quite close together at the "78 ohm" end.
I'm sure you have the formula for parallel-wire-line impedance.

No need for other than a linear taper? Yes, I have Reference Data for Radio
Engineers, fifth edition.


Yours,

Doug Goncz ( ftp://users.aol.com/DGoncz/ )

Read about my physics project at NVCC:
http://groups.google.com/groups?q=dgoncz&scoring=d plus
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in the search box

 

[Doug Goncz]

From: Robert Baer robertbaer@earthlink.net

Perhaps the use of a 300 ohm to 75 ohm bakun would be the solution...

Tinkered with it, and it still has possiblities as the 75 ohm unbalanced line
_is_ sheilded.

Not having much trouble with interference now. Receiving clear stereo at 99.15
MHz.


Yours,

Doug Goncz ( ftp://users.aol.com/DGoncz/ )

Read about my physics project at NVCC:
http://groups.google.com/groups?q=dgoncz&scoring=d plus
"bicycle", "fluorescent", "inverter", "flywheel", "ultracapacitor", etc.
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[Doug Goncz]

Thanks to all participants in this thread.


Yours,

Doug Goncz ( ftp://users.aol.com/DGoncz/ )

Read about my physics project at NVCC:
http://groups.google.com/groups?q=dgoncz&scoring=d plus
"bicycle", "fluorescent", "inverter", "flywheel", "ultracapacitor", etc.
in the search box

 

[Doug Goncz]

From: dgoncz@aol.com.bat.exe ( Doug Goncz ) (Me)

Hello All.

I have a wonderful seven element antenna, highly directional, that is capable
of capturing WHFS from Seven Corners, VA.

And it does now. It's 8:30 PM local time and I've got HFS in stereo!

I slit, laced, folded, and glued the end of the line as described earlier. Then
I melted the spacing insulation down linearly over a distance of half an inch
and stripped the ends of the adjacent wires with heat.

I tidied all the melted bits, fluxed and tinned the now very close together
ends after twisting and straightening them, and soldered them _inside_ the tabs
for the ring and tip connections on the plug, keeping them as close to the same
spacing as possible. Then I applied hot melt polyethylene glue and rolled it
down neatly.

It works well. I am not sure if it's better then before. I added a cigarette
box spacer at the light end of the antenna to get it away from the ceiling
plane. Wow, did that every help. From mono to stereo with that move. Now to get
spacers on the tips to keep them the same distance from the ceiling?

Can I measure the input impedance of the receiver with an ohmmeter or do I need
a signal source of known voltage and an RF ammeter? Would applying the ohmmeter
damage the input? Oh, it's probably coupled with a capacitor. Time for a closer
look at the board.

I found the manual in an old box of receipts.




Yours,

Doug Goncz ( ftp://users.aol.com/DGoncz/ )

Read about my physics project at NVCC:
http://groups.google.com/groups?q=dgoncz&scoring=d plus
"bicycle", "fluorescent", "inverter", "flywheel", "ultracapacitor", etc.
in the search box