Clive,
Sorry for the belated response, and sorry to the list if this was already
suggested. I have had good luck tuning different Moxon antennas according
to the following method.
Model the antenna to get the desired result. Then (in the model) place a
high impedance load (1k inductor, or an open circuit) in the center of the
driven element and put the source in the middle of the reflector. Sweep the
reflector (in the model) to find its resonant frequency and write that down.
Build the antenna as usual except place an insulator in the middle of the
reflector. Then make sure you can open the driven element at the center by
disconnecting one leg from the balun, but still physically present as it
normally would be. Connect your balun and feedline temporarily to the
reflector, across the center insulator, as if you were going to use it as a
dipole. Raise the antenna and then use an analyzer to find the frequency of
lowest swr, which should be the resonant frequency of the reflector. If it
is different from what the model showed it should be, bring the antenna back
down and trim the reflector accordingly. Repeat this until you get the
reflector to resonate at the frequency the model was calling for.
Next, remove the feedline/balun from the reflector and short across the
center insulator. Connect the feedline/balun permanently to the driven
element. And raise the antenna again. Using an analyzer, determine the
resonant frequency of the antenna. If it is not where you want it, lower
the antenna and trim the driven element accordingly. Repeat until you get
the antenna resonance you are looking for.
This is a surefire method, because you end up compensating for any
real-world contingencies such as end insulator effects, insulated vs
uninsulated wire, and objects near the antenna that might interfere with it.
I have used this on 40m field day Moxons where we could only get them up 40
feet or so, and had cars and campers all over the place. An anecdotal, but
compelling observation is that we routinely would place in the top ten or
fifteen nationwide in 5A on field day, with most points coming from 40m CW.
Another observation is that with 100W from New England, we were always able
to own our frequency all night.
Good luck with this.
Dudley - WA1X
Clive wrote...
Message: 1
Date: Sat, 26 Mar 2005 13:46:34 -0000
From: "Clive Whelan" <clive.whelan@btinternet.com>
Subject: Re: [TowerTalk] FW: Moxon rectangles
To: <towertalk@contesting.com>
Message-ID: <NJEKIHLAGJFHHNGLLDFMKECDEAAA.clive.whelan@btinternet.com>
Content-Type: text/plain; charset="us-ascii"
Many thanks to everyone who replied here and in direct email with
suggestions and even an NEC4 model! I hope you will excuse that I haven't
been able to reply to everyone individually, but your help is really
appreciated.
I am now fairly convinced that the problem ( LF shift) is due to the use of
insulated wire, although this remains to be proved in practice. Yes there
are environmental issues, especially at the low height and unfortunately
these are to a more or less extent inescapable. Where I live, no antennas at
all are allowed, though clearly I have some verticals and wires hiding
amongst the trees! The Moxon has to be erected in a position where the
conservation area cops will not be able to see it, and that implies at not
more than 30ft. That is just the price I have to pay for living in a truly
beautiful spot, but I am still coming to terms! I suppose I just wish that
the published literature had made a caveat about the use of insulated wire;
perhaps I missed it but......
I will post further when the problem is finally resolved.
73
Clive
GW3NJW
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|