Spring 2008 Issue
Pitfalls in Stacking Antennas
By Kent Britain, WA5VJB
Whatís wrong with this antenna?
Last time, in the Winter 2008 issue of CQ VHF, I challenged you to tell me what is wrong with the antenna in photo G (in the current column, photo A). I have to admit itís not easy, and the previous owner of this antenna never figured out why it was such a terrible 432-MHz SSB mobile antenna either. It took me a while to see the problem. However, congratulations to W6OAL, N5SRE, and W6TCP, all of whom saw the assembly error. Take a close look at how someone flipped over the bottom element so that the coax connectors would go together more easily. To make the connectors a little easier to attach, the bottom element was flipped. Therefore, one PL259 is pointing up and the other is pointing down. The phase of the two elements was inadvertently reversed.
In figure 1 we have the pattern showing what a normal
set of phased omnidirectional antennas would look like in free space.
Now look at figure 2, in which the phasing is flipped 180 degrees on one
of the antennas. There is now a deep null right at the horizonóa bad
pattern for a mobile antenna. In figure 3 we have an EZNEC simulation of
what the pattern of these out-of-phase antennas would look like when
mounted on a car or truck. See how there is very little signal near the
horizon? The antenna might work for an OSCAR overhead pass, but itís a
lousy pattern for UHF grid hopping.
The winner in the improperly assembled antenna category goes to a Cushcraft Jr. Boomer for 144 MHz I picked up at a garage sale, of all places. The XYL running the sale said that the owner had not been happy with the antenna. I later noticed that some of the directors were longer than some of the reflectors. Thatís not the usual arrangement for Yagi elements. Out came the assembly instructions for the Jr. Boomer I already had in the air. I did a quick check, and out of the 14 elements, including the driven element which only fits in one hole, only three elements were in the right holes. Yes, it does make a difference which elements go where. It looked as if that antenna had been randomly assembled. I then reshuffled the elements and stacked it with my existing Jr. Boomer. I made several EME contacts, a few meteor-scatter QSOs, and a contact with W5LFL on the space shuttle with that pair of Jr. Boomers. All the QSOs were CW and SSB (no JT65 back then). Well, W5LFL was on FM, but that one was line-of-sight. But I digress.
Yes, it is possible to add an extra 1/2 wavelength of coax to one side of the phasing lines and flip the flip, getting both antennas back to the same phase, but this usually is not recommended.
Photo B is an example of this flip of a flip. It is a sector patch antenna I made for a client. Note that one patch is fed from the bottom and one from the top, so the patches are naturally 180 degrees out of phase. You can also see how the coax connector is not exactly half way between the two patches. To one patch I have a 1/4-wave matching line, and to the other patch I have a 3/4-wave matching line. Now the antennas are matched and back in phase.
Photo C is a more typical example of how a pair of
patch antennas would be phased. However, the flip of the flip gave the
final size, gains, pattern, and polarization the client wanted.
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