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THE AM BULLETIN BOARD => Technical Forum => Topic started by: KA3EKH on March 17, 2023, 09:49:28 AM

Title: Antenna for Field Use
Post by: KA3EKH on March 17, 2023, 09:49:28 AM
Every year at the Hamvention there is a low power 3885 AM Military radio net on Saturdays at noon, we all try to use handheld and backpack military radios from WW2 and talk to each other at that time. Lot of things like the GRC-9, DAV, MAB, Pogo Stick radios and maybe one of the largest gatherings of working BC-611 “Handy Talkies” in the world. I usually set up something larger and this year am intending on using a 1936 RCA receiver along with an ARC-5 transmitter all run from a modulator power supply that I put together so it all runs on a twelve-volt car battery.
I have a seven-foot rod antenna with a base coil that’s maybe a fifty turn B&W coil mounted on a wooden pole that I put up for whatever radio I am running at that time.
My question is what way is better for driving the base coil on the antenna? Figure A where the shield of the coax is grounded to the coax or figure B where the coil is in series and the shield of the coax floats at the feed point?
I know that the transmitter I am using was never designed to run into a fifty Ohm load but I do all my testing and set up into a fifty Ohm load and with the internal loading coil set to zero on the transmitter I can see a good five or ten watts from the transmitter into fifty Ohms and I am using maybe twenty feet of coax between the output of the transmitter.
The configuration in A tends to be more stable and has the advantage of being able to be tuned with a VNA prior to transmitting so somehow think that’s best.
Like to see what others recommend. And as a footnote I also do a separate ground at the transmitter and like to tie the ground to other metal items like the trucks body or tent frames at the event as a counterpoise.

Title: Re: Antenna for Field Use
Post by: Opcom on March 17, 2023, 07:48:36 PM
The first would seem to be more of a tapped transformer and would seem to adust the resistance (assuming reactance of the load is zero or has been tuned out by a network.

The second seems to be for tuning out reactance.

A reference to my reply is based on the BC-939 tuning unit used with the BC-610.

L5 is the autotransformer to match the transmitter's output impedance to the reactance-tuning portion of the unit, and L6 or L44 plus C30 or C22 are the portion that tunes out the reactance.

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