[TowerTalk] Driv Elem Tuning

[Eric Gustafson]

Hi Ken, Ron,

You can feel free to make adjustments to the driven element to
bring its match into range.  Absent a _really_ large change to
the dimensions of the DE or any change to its location on the
boom, there will be virtually no effect on the Gain and F/B
ratio.

I would not fool around with the other elements unless you have a
means to verify the resulting pattern once it is up in the air.
At any rate, this will probably be unnecessary on a commercially
made antenna unless you are trying to compensate for interaction
with other antennas in close proximity.

Compensation for close by antenna interaction is potentially one
of the most useful applications for the NEC based modeling
programs.  Because in this case, you really don't need to worry
about the model geting the absolute lengths of the parasitic
elements exactly correct.  If this is what I was up to, I would:

1.  Make a model of the antenna and get it working like the
    manufacturer claims (or as close as possible).

2.  Make a working model of the proximate antenna.

3.  Place them as intended in practice in the model relative to
    one another.

4.  Retune the parasitic elements on the antenna under test until
    the performance returns.

5.  Note the magnitude and direction of the _changes_ to the
    element lengths and apply them to the antenna.

6.  Put the system up and retune the DE for best match.

This should save a LOT of climbing and considerably reduce the
exposure of humans to dangerous heights in order to get this job
done.

73, Eric  N7CL



>Date: Tue, 01 Sep 1998 17:31:52 -0400
>From: "Kenneth D. Grimm" <grimm@lynchburg.net>
>
>Ron Chambers wrote:
>> 
>> The practical side of assembling, building, erecting and
>> tuning beams that are mounted on non-crank-up/down towers
>> presents a problem. You get the thing all assembled per the
>> design/manual, take it all the way to 60, 70, 80, or even
>> 180ft with the help of several local OM's, put it on the
>> analyzer and voila', the "tuning" is off a little. You could
>> beg all your help to stay, go thru the procedure of getting it
>> back down the tower and adjusting, then back up, etc....maybe
>> just to do it all over again....ad infinitum (maybe I could be
>> a lawyer)......
>> 
>> So my question is, what are the REASONABLE limits, say, if you
>> have a 3 element beam (monobander? tribander?), of adjusting
>> only the driven element length (which you can usually get to
>> on top of the tower)and not diminish the ability of the
>> antenna to achieve what it's supposed to?  ...The point being
>> that you are not going to touch the length of the reflector or
>> director....(Yes, this was questioned after K5MR repaired his
>> full-size 3 element 40M at 170ft and it was 'off' a little, we
>> did NOT want to take that thing down...)
>
>What a great question!  I hope someone who is into modeling will
>take the time to come up with an answer.  Obviously, the correct
>thing to do is to change all elements, but what is the magnitude
>of the penalty that you pay for just tuning the driven element?
>
>I too, have been caught with a very large antenna erected
>according to the manufacturers dimensions and which was a
>"little" off and wondered if I really had to take the whole
>thing down or whether I could "get by" with simply adjusting the
>driven element by a wee bit.
>
>For example let's take a 20 mx beam ...the ubiquitous HyGain
>204BA.  You put the thing together using HyGain's measurements
>for the Phone settings and on top of the tower is resonates in
>the middle of the CW band.  What happens to the pattern, gain,
>f/b etc. if you shorten the driven element by 3 inches and leave
>the other element lengths the same?
>
>73,
>-- 
>Ken K4XL
>grimm@lynchburg.net
>BoatAnchor Manual Archive - ftp://bama.sbc.edu

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