Hello -

I am setting up an antenna for receive only SW listening purposes - to be used with a SDR and several old boat anchor radios.

I am building a new-old-stock Mosley SWL-7 trap dipole antenna I bought on eBay. The antenna comes with 100 feet of 75 ohm balanced-line for the feed line (very thin/small) and a direct connect between antenna and feed line at the center, no balun.

The antenna by design is resonant on specific SW broadcast bands.

My question is - do I need to use the full 100 feet of feed-line to keep the antenna resonant, or can I shorten the feed line to fit my space without impacting antenna resonance?

I believe that if I had a balun and coax, the feed line length would only matter due to loss of signal strength on longer coax runs, but with the balanced line and no balun, I am concerned with changing the antenna's characteristics if I shorten the line.

Please let me know.

Thanks,

John

On Wed, 24 Jan 2018 08:33:55 -0800, johndusek3 wrote:

+AD4 My question is - do I need to use the full 100 feet of feed-line to keep
+AD4 the antenna resonant, or can I shorten the feed line to fit my space
+AD4 without impacting antenna resonance?

The short answer is -- shorten the lead in to whatever is convenient and
you'll be fine.

That's assuming that the radio end of the lead in matches the impedance
of the coax. If the radios have coax connectors they're probably close
enough.

Older boatanchor radios, say from before 1950, commonly had higher
impedance antenna inputs, designed for end fed wire antennas. If you
have such a radio, you might need a balun or matching unit for best
performance. But a mismatch won't damage anything.

On Wed, 24 Jan 2018 08:33:55 -0800, johndusek3 wrote:

+AD4
The antenna comes with 100 feet of 75 ohm balanced-line for
+AD4 the feed line (very thin/small)

Sorry for my brain fart reply. Something in my head said +ACI-75 ohms +AD0
COAX+ACI and I didn't read carefully.

Anyway, the main point is the same. If a transmission line sees an
impedance at each end equal to the characteristic impedance of
transmission line then the effects of changes in length will be minimal.

And here's some notes from the +ACI-Be Aware, But Don't Overthink This+ACI
department:

Do your radios have a 75 ohm antenna impedance? That will make a
difference to the transmission line.

Also, the height above ground is going to have an effect on both the
antenna's resonant frequency and impedance. There's some graphs on this
page, computed at 7.1 Mhz:

http://www.comportco.com/+AH4-w5alt/....php?pg+AD0-13

I expect you don't have many choices about where to put the antenna.
Higher is better than lower.

Getting the antenna out of the house and farther away from the electronic
noisemakers is the biggest deal here, everything else is small potatoes.