[50mhz] Cushcraft A50-3S Question

[Bill W5WVO]

This is a "me too" post.  :-)  Yes, what Jonesy said.

If at all possible, get or borrow an MFJ-259B (or newer) antenna analyzer. 
This will allow you to adjust the antenna for true resonance (reactance = 0, 
resistance = 50) at your desired frequency. There just ain't any easy to do 
that without such an instrument.

Connect it as close to the antenna coax connector as possible -- a one-foot 
jumper is good. Make your trial adjustment to the antenna, then STEP BACK away 
from the antenna until the readings stop changing, then note the readings. 
Repeat. If you have the antenna mounted on a plastic or wooden chair or ladder 
(NO metal) and pointing straight up at the sky, you shouldn't have to step 
back too far to get out of the field.

You will probably need to manipulate three things: the driven element length 
(with exactly equal adjustments on each side of center), the gamma match tap 
point on the driven element, and the gamma match capacitance (concentric 
rod/tube overlap). Do NOT adjust the length or position of either the 
reflector or the director.

If you have never done this before, take your time, and you'll have a LOT OF 
FUN!!  :-)
Bill W5WVO


Marvin L. Jones wrote:
> On Mon, 11 Feb 2008, George McCrary wrote:
>
>> I'm getting ready to put a used (good condition) A50-3S up at about
>> 57 feet. I currently have the antenna on an 8 foot mast leaning
>> against a pine tree.
>>
>> I've set the antenna up for resonance at 50.0 MHz according the
>> measurements in the manual. When I measure the VSWR I actually get a
>> dip at around 50.8 MHZ where the VSWR is about 1:1. The VSWR at 50.0
>> is almost 3:1.
>>
>> Are the measurements in the manual that unreliable, or is the
>> antenna being affected by it's proximity to the tree and ground?
>
> Maybe.  Probably.  No doubt.
>
> Using a tall wooden or fiberglass step-ladder -- full clear of any
> nearby structures, clamp the mast to the top platform with the antenna
> pointing STRAIGHT UP.
>
> Now (re)do your SWR measurements -- and insure that _you_ are nowhere
> inside the antenna's capture area.
>
> Jonesy