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Author Topic: Bi-square antennas and remote-control switches  (Read 1623 times)

Posts: 60


« on: June 18, 2007, 02:35:55 AM »

I was looking through my 20th edition ARRL Antenna Book and saw the bi-square antenna at the end of chapter 8.  I have a couple of questions about the antenna, though.  

How exactly do I feed the two elements of the antenna out of phase?  If it's delay line I have to use, I'll need two different lengths which will be a little awkward.

How can I remotely switch between two arrays *and* close the top of the array when necessary?  Looks like I'll need two relays -- a double-pole double-throw relay for selecting arrays and a single-pole double-throw relay for switching between full-loop and bi-square mode.  With two relays, I can't use the feedline to drive the relays.  Can I run a length of Ethernet cable parallel to the coax feedline and use the twisted pairs in the cable to drive a pair of relays?  If so, how do I wire the relays?  Anyone have any pointers or examples on how to do this?

Finally, the antenna in the book uses 300-ohm feedline, and all I have is good 50-ohm line.  Should I use a 6:1 balun, or should I get some 300-ohm feedline and connect it to the open-wire interface on my antenna tuner?

Thanks in advance for your help.  I really wish I'd thought of this a month before Field Day and maybe it'd be up on the air for this weekend.  Ah well. :-)

Posts: 17480

« Reply #1 on: June 18, 2007, 10:03:24 PM »

The simplest way to feed the two sides out of phase is to connect them
to the two wires of a feedline.  The current/voltage in one side is 180
degrees out of phase with the other.  Commonly this would be done
with some sort of high impedance balanced line since the impedance is
pretty high.  A quarter wavelength of 300 ohm line might step down the
impedance to something close to 50 ohms (if the actual impedance at
the feedpoint is around 2000 ohms.)  Direct feed with 50 ohm cable
without the matching section would have an SWR of 40 : 1, which
generally isn't a good idea.   One solution for single-band use would be
a shorted quarter wave matching stub at the feedpoint, with the coax
tapped onto it at the right point to match the impedance.  But this
probably won't work for two-band use.

The simplest method (the one that I used) is to use twinlead or open
wire line (commercial or home made) with a tuner at the rig end.  You
may have to adjust the feedline length to get a matchable impedance
on both bands.

When I built a dual-band bisquare for 10/20m  I used a quarter wave
section of twinlead at the top to change the configuration for each
band.  By using a quarter wavelength of twinlead open on the bottom,
it acted as a short circuit on 20m and an open circuit on 10m.

Posts: 60


« Reply #2 on: June 18, 2007, 10:48:28 PM »

If all I have to do to feed them out of phase is to use 300ohm ladder line instead of coax, that's totally fine with me.  That would be an awesome way to solve my problem.

You mentioned using a quarter-wavelength section of twinlead or ladder line as what amounts to a trap.  Is that a quarter-wavelength of the longer wavelength or the shorter wavelength?

Posts: 17480

« Reply #3 on: June 19, 2007, 06:14:48 PM »

A quarter wavelength line acts as an impedance transformer, so a short
circuit at one end looks like an open at the other, and vice versa.  A half
wavelength line repeats the terminating impedance at the other end.
So by using a quarter wavelength line on 20m it looks like a short
circuit, while on 10m where the line is a half wavelength it looks like
an open circuit, which is exactly what you want in that situation.

So, to answer your question, a half wavelength on the design frequency
of the half square, and a quarter wavelength on half the frequency
where you are using the antenna as a full wave loop.
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