Tom,
I have built quite a few wire beams. And tuned them in a variety of ways;
some pretty complex and some pretty simple.
If you want to avoid doing any modeling, I would suggest you do it this way.
It's a bit iterative but it will work.
You need something that will show reactance. That's an MFJ 259 or some
such. Find out where x=0. That's your resonant point (which may or may not
be the same as your lowest SWR point) - forget about SWR for the moment.
Next I would drop the reflector. It's not going to give you much and adds a
tuning complexity that is hard to overcome without gear. So you have a DE
and a DIR. The DE needs to be fed at a right angle to the vertical dipole -
and it needs a serious coax choke to keep the feedline out of the mix.
Make the length of the DIR about 10% shorter than the DE. Now start
shortening the length of the DIR. Adjust the MFJ to find X=0. What we are
trying to do is to get a DIR length so that the feedpoint is around 25 ohms
R at X=0.
Once you have this feedpoint value, you can be assured the beam will have a
tight coupling between the two elements.
Look at that X=0 frequency; it's going to be off from what you want the beam
to be on. To trim the frequency, lengthen or shorten the two elements by
the SAME AMOUNT, and that will push the X=0 point in frequency.
Once you have X=0 at your target operation frequency, you can add a hairpin
(basically a short hunk of wire over the feedpoint) to give a 50 ohm match.
At that point, you will have a pretty decent operating vertical. And you
won't have to do any analysis or modeling.
73/jeff/ac0c
www.ac0c.com
alpha-charlie-zero-charlie
-----Original Message-----
From: Tom Osborne
Sent: Friday, May 10, 2013 8:32 PM
To: Towertalk
Subject: [TowerTalk] 3 el wire beam
Hi All
Been playing with an antenna here.
Some time back I put up a vertical dipole for 40 meters. Worked OK. A
little later I added a director about 15 feet in front. Today I put up a
reflector about the same distance behind it.
What has me wondering is that putting the other 2 elements on it didn't
change anything on the SWR. Does that mean they aren't working?
Seems like something should have changed on the SWR if the other elements
were working. Thanks and 73
Tom W7WHY
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