[Milsurplus] Forest Service SPF radio detail (Windom)

[Sean Kelly]

I think the salient point when using twin lead is that the tuner at the transmitter/transceiver does something to keep the reflected wave voltage and current manageable every time it returns from the antenna and that the RF energy can be reflected back from the antenna several times, each time reflected back from the tuner towards the antenna.

So, say the SWR is...3:1 and...

OK, plan "B" - because the online calculator.

Say the VSWR is  1:2.6  with the transmatch at the radio.

30 watts forward power
2 watts reverse power

The feedline deducts from the forward power, say RG-58 and 3.3 dB per 100' at 50 MHz. Out of the 15 watts that arrives at the antenna, 2 watts is reflected back to the transmatch. What I was told is that it then reflects again towards the antenna. This time 1 watt arrives (veerry quickly, packet radio fans) at the antenna and 0.9 watts enters the antenna and about 70 milliwatts is reflected back to the transmatch. At the end of each trip along the feedline, the percentage of reflected power just keeps bouncing back and forth, becoming less each time. Third trip: 40 milliwatts arrives at the antenna, 3 milliwatts is reflected and half of that arrives back at the transmatch. The 40 milliwatts arrives 5 lengths of the cable run "later" than the original 15 watts. With a velocity factor of 66% that would be ~166 meters/speed of light in free space * 1.5 =  2 micro-seconds (not enough to change the shape of the pulses of a digital mode enough to disturb its synchronization, I'm sure).

One disadvantage of ladder line is that you can't place it near metal while you are using it. But a big advantage is that the loss is so low, like 1 dB per 100' of 300 ohm twin lead:

https://www.rfcafe.com/references/electronics-world/loss-figures-300-ohm-twin-lead-january-1965-electronics-world.htm
[https://www.rfcafe.com/references/electronics-world/images/loss-300-ohm-twin-lead-electronics-world-jan-1965-1.jpg]<https://www.rfcafe.com/references/electronics-world/loss-figures-300-ohm-twin-lead-january-1965-electronics-world.htm>
Loss Figures for 300-Ohm Twin-Lead, January 1965 Electronics World - RF Cafe<https://www.rfcafe.com/references/electronics-world/loss-figures-300-ohm-twin-lead-january-1965-electronics-world.htm>
The last time I remember using 300-ohm twin-lead transmission cable for a television antenna is about 20 years ago when I lived in a fairly rural area and had a VHF/UHF aerial mounted on a 20-foot tower with a rotator.
www.rfcafe.com
So more RF energy arrives at the antenna each time, less loss so you can have a greater mismatch and transmit the same amount of the RF. It's not without limits although the Zepp Antenna is end fed by ladder line plus the Double Zepp is s wavelength long antenna center fed by ladder line so the feedpoint is a voltage node, in both cases the impedance might be 5,000 Ohms. In the case of the single Zepp, since no or very little current flows at the end/feedpoint, it's OK that the second conductor is not connected - my understanding is that the current flows back and forth in the middle, same as a half-wavelength dipole.

https://youtu.be/Js-gEZK9A-E?t=358
[https://i.ytimg.com/vi/Js-gEZK9A-E/maxresdefault.jpg]<https://youtu.be/Js-gEZK9A-E?t=358>
How a Transmatch Works<https://youtu.be/Js-gEZK9A-E?t=358>
Antenna tuners eliminate reactance and match resistance. http://www.sciencewriter.net
youtu.be
Of course, in the real world, antennas are compromises and Hugh, seems like you have almost one half of a perfect ground closeby 🙂

http://webclass.org/k5ijb/antennas/Windom_K4ABT-measurements.htm
Windom antenna - Webclass.org<http://webclass.org/k5ijb/antennas/Windom_K4ABT-measurements.htm>
The WINDOM Antenna G. E. "Buck" Rogers Sr - K4ABT. In September of 1949, I was tired of climbing poles and trees to move, remove, add, or change my single-band HF antenna's.
webclass.org
Sean
~~~~~
Subject: Re: [Milsurplus] Forest Service SPF radio detail (Windom)

That seems like a good point. But then, my question assumes "perfect ground". I would guess the high impedance antennas the F.S.
used for portable equipment was to overcome to some degree less than good ground situations.
-Hue

>When I started out in this game some 60 years ago the 2-wire-fed Windom
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