Hello Bob,
Some recent years back, with some access to acreage, I thought of doing a
rhombic antenna, so learned a bit about them.
To get significant gain, you need quite long leg lengths, in terms of
wavelengths at the frequency you plan to operate on. 3 and more
wavelengths per leg is desirable. For 20 meters that comes to, say 200 ft
per leg, or a little shy of 400 ft point to point. As I recall
for maximum gain, there is an optimum internal angle for the vertices.
Something like 68 to 70 degrees interior angle for the major axis corners.
This angle is tweaked a bit depending on how many wavelenghs per leg are
used, and how high above the ground the antenna is. Generally, as the
number of wavengths per leg increases, the optimum rhombic gets narrower.
If you put a non inductive resistive load on the far end of the rhombic, the
reverse lobe is partially cancelled, also the rhombic feed impedance smooths
out considerably over a very wide range of frequencies. providing reasonable
SWR. Dont worry too much about SWR, since you are feeding with 400 to 600
ohm low loss parallel wire feed, and you can tune out any impedance
mismatch.
The front to back or front to side numbers describe a pattern quite
different from your typical Yagi Antenna. The pattern of the rhombic often
has many needle like lobes straddling the main lobe, and with considerable
strength compared to the main lobe. Lots of narrow energy leaks.
>From the fundamental design frequency, as you go up in frequency, you
eventually get a pattern that splits the main lobe in two, straddling
closely the main axis, with a null right at the axis. I dont recall the
usable frequency span a rhombic can work well on; heavily dependent on
wheather you iinstall a load resistor at the far end.
The load resistor at the far end has to have a power rating of 1/3 to 1/2
the feedpoint power, and a value somewhere around 600 ohms. Some military
deisgns make the load resistance not from a resistor, but from slowly
tapered, high loss parallel transmission line made out of lossy stainless
steel or nichrome wire, thereby dissapating a lot of the power in the lossy
line. At the end of the lossy line, you may or may not need to put a
resistor there, and if you do, its power can be considerably lower than a
discrete reistor at the wire junction. This lossy line, may be in the order
of 100 to 200 ft long, and is folded back towards the center of the rhombic.
I used to work a New Zealand Station, something like ZL2MAN evenings who ran
a rhombic pointed to the west coast, and he was devastatingly strong. He
also had a TH 6 for comparison, and the TH6 was usually profoundly weaker
than his rhombic.
My bet is you need sturdy, guyed poles for the corners, as to keep the wire
sag on 200 ft spans low, requires a lot of tension.
A master design book on Rhombics, and other large HF antennas, was written
by Laporte in the early 50s, and is a very good read/reference. A complete
(every page) image file of the book is located somewhere on the web.
If you Google Search Rhombic +W6AM you will see some history and aerial
images of the greatest ham radio rhombic farm ever built, by Don Wallace, In
LA...really incredible.
Good luck on your rhombic, and I am sure there are others here, who can be
helpful to you as/if you go forward on a rhombic...
73, DX, de Pat AA6EG aa6eg@hotmail.com;
Skype: Sparky599
>From: "bob" <wf3h@vsswireless.net>
>To: <towertalk@contesting.com>
>Subject: [TowerTalk] rhombic antenna
>Date: Sat, 29 Jul 2006 22:17:55 -0500
>
>I'm thinking of erecting a rhombic for 20M. From what I've read, the angle
>at the feed point, and the angle at the legs determines the F/B and F/S
>ratio. Also, it seems to be best to feed it with ladder line. In addition,
>folks who've done it say it can be used from 1/2 to 2X the resonant
>frequency with great effectiveness.
>
>The biggest issue is support it. I don't have a bunch of telephone poles,
>and am thinking of getting a few of the 'push up' masts that could be used
>around the property, but am not sure if these will handle the weight of the
>wire and the intermittent wind loading here in Dallas, TX.
>
>A few years ago, a bunch of guys and I from the Frankford Radio Club
>visited WOO's receive site at Manahawkin, NJ where they had dozens of
>rhombics for maritime comms. They obviously worked great.
>
>So what say you Talkians? Experiences? Ideas?
>
>Thanks all!
>
>Bob/WF3H
>
>
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