E-Mail to N2PK, et al. on my Bi-Plane antenna for 20 Meters

From: "Matz John" KB9II
To: "Kiciak, Paul" N2PK, et al.
Subject: Bi-Plane Antenna
Date: Tuesday, July 03, 2001 10:34 AM

Hi Paul,

I'll be putting some pictures of the antenna and installation on my
web-page, probably tomorrow.  I have not had much chance to write things up,
but here's some basic info off the top of my head..

The Bi-Plane is in Antennex.com.  It was developed by "Old George" Sharp, KC5MU.  There were a couple of articles about it in the last year, one under Ham Workshop, I think, and one authored by Jef Verborgt.  I took the info there and ran with it.  

My version looks kind of like a squashed biplane wing assembly.  It is
made of up out of four plates, upper-lower, left-right, each 12" x 12"
originally, with a one-inch flange turned up in front and back for rigidity.
The "wings" are only about half an inch apart.  The wings are spaced about
apart in the center.  The wings are "cross-coupled" by coils from the
upper-left to lower-right and lower-left to upper-right, like an X.  The
feed coax drives the lower-left and lower-right plates.  The coax has a
6 turns, 4" diameter, wound in it, right below the antenna.  A
picture really helps. So ... it is about 10" x 36" x 3" high for 20m.

Tune-up is fairly simple.  The antenna is basically two series L-R-C
circuits in parallel across the feedpoint.  The key is the C's are very
large, which makes the L's very small.  The R's are made up of the coil
losses and the radiation resistance.  Since the L's are small, the coil R's
are too, and efficiency should be better.  If the two LRC's are both tuned
to the same frequency, the feedpoint impedance is very low and match is
But if they are stagger-tuned, one high and one low in frequency,
feedpoint impedance can be made to be 50 ohms resistive, so match is good.
Since C is large, and L is small, bandwidth is wider than an Isotron or
short dipole.  There may be some "compensating" from the other tuned circuit
going on too.

But the big unknown is efficiency -- would it be any better than an Isotron,
etc.?  Well ... it's not too bad.  I have my half-rhombic up at 22 feet also
25 feet away.  I can A/B the two on receive, and after comparing six
stations, they are basically identical.  I have used my 2m beam as a sense
antenna at 30 feet up, 25 feet from each.  The signal strengths with a
milliwatt source were the same (+-2 dB, depending on where I was in the
band).  All I have proved is that this Bi-Plane antenna is no worse than my
half-rhombic, which should be slightly worse than a dipole.

Sources of error ... well, the coax feed to it could be radiating as a
vertical, the coax to my 2m beam could be picking up signal at 14 MHz, the
half-rhombic may be re-radiating signals ... yes, I have not eliminated them
yet.  I'm still in the testing stage.

But ... QSO's with IL, TX, AL, AZ, Germany, FL twice ... and W1AW runs
S9+15dB on RX here ... it seems to be working.  

More to come ...
John Matz KB9II