[Antennas] Multiple antennas

2002年6月08日 21:45:43 -0500

Just a couple of observations, Bob. First of all, I don't quite see how
three horizontal antennas can be arranged - with each in a straight line -
to form a diamond. A triangle perhaps?
Second, with the antennas that close there will be considerable mutual
coupling among them, regardless of which one (or ones) is connected by the
switch, so the tuning situation has to take that into account.
For that and other reasons, I would recommend against using a 4:1 balun.
Rather, I would use a 1:1 current or choke balun since the entire purpose
of it is to make the transition from ladder line to coax and thus to
minimize common-mode current on the outer braid. It is unlikely that a 4:1
transformation will be of any benefit since the EDZ is a non-resonant
antenna and the line inpute impedance will be complex regardless of line
length. The 1:1 balun is much more tolerant of termination impedance since
it amounts only to an r-f choke formed on the outer braid of the coax.
Before putting up that much wire - man, what a lot of work! - I would model
the entire setup with EZNEC and try to pin down what the driving-point
impedances look like, how the antennas interact, what patterns result from
various driving combinations, etc. I think that this is the only way that
you can predict the outcome and get a handle on some of the problems that
can arise with a complex array like that.
I use two 20-meter EDZ antennas but they are mounted at right angles and
seem to have little effect on one another. I, too, use the same ladderline
lengths and then W2DU bead baluns to same coax lengths into the shack and
to the tuner. Once tuned, I can switch from one to the other for a 90 deg
direction shift.
If your operation is restricted to 40 meters, then I would expect the
situation to be less complicated. But, I still recommend that you do the
modelling before stringing wire.
A final note: the coax from the antenna(s) through the coax switch and on
back to the shack may be operating under a large SWR with attendant
increased loss. As long as the SWR is in the order of 5:1 or less and if
the coax is 100 ft or less, then the added loss due to load mismatch is
probably tolerable. But, this can be another gotcha, which is yet another
reason to analyze the entire system, including estimating the coax SWR. In
any event, I would plan to run RG-213 or comparable regardless of the power
level you operate. You should use a solid dielectric cable instead of foam
to obtain a higher voltage rating.
73/72/oo, George W5YR - the Yellow Rose of Texas 
Fairview, TX 30 mi NE of Dallas in Collin county EM13qe 
Amateur Radio W5YR, in the 56th year and it just keeps getting better!
QRP-L 1373 NETXQRP 6 SOC 262 COG 8 FPQRP 404 TEN-X 11771 I-LINK 11735
Icom IC-756PRO #02121 Kachina 505 DSP #91900556 Icom IC-765 #02437
Robert Wilder wrote:
>> I would like to pose a question. I have room to put up three (3)
> identical double extended Zepp antennas.for 40 meters They would be 120
> degrees from each other in the shape of a diamond. Each antenna will be
> fed with an identical length of ladder line terminated in a 4:1
> balun. Each balun will be fed with about three feet of coax to an antenna
> selection switch (like the Ameritron RCS-8V) and in turn to a single coax
> lead back to an antenna tuner in the shack. I know that this would give
> me identical patterns but 120 degrees apart which pretty well cover the
> world. Now the question. If the switch can be made to select a pair of
> these antennas or even all three at one time, what will happen? That
> is: Pattern(s), Impedance, radiation resistance, etc. Contact me off list
> or if you feel others might be interested via the list.