[50mhz] Cushcraft A50-3S Question

[Bill W5WVO]

One more admonition: If you measure your SWR in the shack (close to the 
transceiver), you will not actually be able to adjust the ANTENNA for 
resonance, or even for a flat SWR, because you will be including the feedline 
in the equation. Any adjustment you find that gives you a flat SWR at the 
transceiver will almost certainly not be flat at the antenna itself, and it 
will be sensitive to the length of the coax you're going through. This is 
because of the presence of reactance. You want to get the reactance to zero, 
and then the length of the feedline or the position of the SWR meter doesn't 
matter. When adjusting a yagi, always put whatever measurement device you use 
as close to the antenna feedpoint as possible.

Bill W5WVO


Bill W5WVO wrote:
> 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