Hi all,
newbie question here...sorry if its covered elsewhere. My radio has a
clip terminal for an ext. antenna, and wants to see 500 ohms. I want to
put a transformer at the longwire for the coax feeder to show it 50 ohms
for the trip into the house...will it work if I put another transformer
in the house to get it back to hi-z?
The transformers will be home made ferrite coil jobs as described in
John Bryants excellent article.
TIA
mike

mw wrote:
Hi all,
newbie question here...sorry if its covered elsewhere. My radio has a
clip terminal for an ext. antenna, and wants to see 500 ohms. I want to
put a transformer at the longwire for the coax feeder to show it 50 ohms
for the trip into the house...

How do you know the impedance of your longwire is 500 ohms?
If it is unterminated at the end, the impedance is definitely
not 500 ohms, except by complete accident.
--
73, Cecil http://www.qsl.net/w5dxp


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"mw" wrote in message
...
Hi all,
newbie question here...sorry if its covered elsewhere. My radio has a
clip terminal for an ext. antenna, and wants to see 500 ohms. I want to
put a transformer at the longwire for the coax feeder to show it 50 ohms
for the trip into the house...will it work if I put another transformer
in the house to get it back to hi-z?
The transformers will be home made ferrite coil jobs as described in
John Bryants excellent article.
TIA
mike

Just hook the longwire to the radio directly at first.
Then run a ground wire to the [gnd] terminal.
(assuming its receive only).
You can also play around with an antenna tuner too - pick one up at a
hamfest for a few bucks.
But its also fun to play with xfrmrs too! You didn't mention the freq you're
listening to or how long the wire was, so you don't really know the
antenna's impedence, so an antenna tuner looks better and better.
Have fun!

thanks for the input!

the antenna I ve started in a 60 ft. 14g. copper wire, up in the trees
away from the house. I,d like to feed this into the house with rg58. my
radio tunes continuosly from 3 to 30 K, but I have also been mucking
around with some crystal sets which are more limited.

I,ve read that the antenna impedance is likely to be around 450 ohms,
which would be fine staight to the antenna connection on the receiver I
guess, but I would like to use the coax to protect the signal from all
nasty stuff my house generates (and to protect the house from the
potentially nasty stuff my antenna might!). So I would like the signal
to go from 450-ish to 50 and back to around 500...

Thanks again...and dont worry about me not having fun...
Mike


Hal Rosser wrote:
"mw" wrote in message
...

Hi all,
newbie question here...sorry if its covered elsewhere. My radio has a
clip terminal for an ext. antenna, and wants to see 500 ohms. I want to
put a transformer at the longwire for the coax feeder to show it 50 ohms
for the trip into the house...will it work if I put another transformer
in the house to get it back to hi-z?
The transformers will be home made ferrite coil jobs as described in
John Bryants excellent article.
TIA
mike


Just hook the longwire to the radio directly at first.
Then run a ground wire to the [gnd] terminal.
(assuming its receive only).
You can also play around with an antenna tuner too - pick one up at a
hamfest for a few bucks.
But its also fun to play with xfrmrs too! You didn't mention the freq you're
listening to or how long the wire was, so you don't really know the
antenna's impedence, so an antenna tuner looks better and better.
Have fun!

Mike,
A 'long wire' (really a 'random wire', but that's being
picky) can have an input impedance all over the place depending
on the frequency of use. Most receivers just aren't too
picky about it, so try it and see what you get in reception.
You can always experiment with antenna length or matching
systems to improve things, but in most cases, they aren't
going to make a lot of difference.
'Doc

'Doc wrote:
Mike,
A 'long wire' (really a 'random wire', but that's being
picky) can have an input impedance all over the place depending
on the frequency of use. Most receivers just aren't too
picky about it, so try it and see what you get in reception.
You can always experiment with antenna length or matching
systems to improve things, but in most cases, they aren't
going to make a lot of difference.
'Doc

Ive been reading up a bit more (and I see what you mean...Im not sure
the missus would be keen on a real long wire in the backyard!) and I
think Ill play around with some DIY antenna tuners to see whats what. I
was mainly concerned with signal loss due to load mismatch if I use coax
to get the signal into the basement for the wire.
thanks again
Mike

Im not sure the missus would be keen on a real long wire in the
backyard!)

What a WIMP!!!

Ron

Mike wrote:
"I was mainly concerned with signal loss due to load mismatch if I use
coax to get the signal into the basement for the wire."

A versatile tuner allows a match between antenna and radio under most
conditions. It does not prevent an unshielded and imbalanced line from
picking up noise on the path to the radio. Coax may avoid noise pickup.

Autotransformers at each end of the coax can give high-impedance match
at the antenna and at the radio. Such transformers are used to match
Beverage Antennas to coax.. See the "ARRL Antenna Book" or ON4UN`s
"Low-Band DXing" for ferrite-winding details.

An air-core transformer at each end of the coax can step up / down the
impedance too. Suppose you want to match 450 ohms to 50 ohms. The
impedance ratio is 9 to 1. The turns ratio is 3 to 1.

The reactance of the primary may ne chosen as about 10 times the 450
ohms or 550 ohms of the antenna. About 500 microhenries (0.5 mhy) would
work. My ARRL L/C/F Calculator shows the number of turns per inch, wire
size, coil diameter, and coil length needed to make the coil.. There`s
nothing critical about it. Its inductance was an arbitrary choice.

The coils are tapped up about 1/3 of the turns from the cold (grounded)
end of the coil, for the coax connection. The full coil is used across
the antenna and across the receiver input.

If you have no noise, a single wire connecting your receiver to the
antenna will likely grab more signal and lose less of it on its way to
the receiver.

I`ve used the transformers on each end of the low-impedance line
(twisted pair)) to the receiver. This system was part of a 1938 GE
"V-Doublet" antenna kit. It was a quiet all-band signal grabber.

Best regards, Richarrd Harrison, KB5WZI

You would get a lot more useful answers on rec.radio.shortwave.

"m" wrote in message
news
thanks for the input!

the antenna I ve started in a 60 ft. 14g. copper wire, up in the trees
away from the house. I,d like to feed this into the house with rg58. my
radio tunes continuosly from 3 to 30 K, but I have also been mucking
around with some crystal sets which are more limited.

I,ve read that the antenna impedance is likely to be around 450 ohms,
which would be fine staight to the antenna connection on the receiver I
guess, but I would like to use the coax to protect the signal from all
nasty stuff my house generates (and to protect the house from the
potentially nasty stuff my antenna might!). So I would like the signal
to go from 450-ish to 50 and back to around 500...

Thanks again...and dont worry about me not having fun...
Mike