[TowerTalk] Rhombic for Field Day

Tom Rauch W8JI@contesting.com
Fri, 21 May 1999 11:11:48 -0400

Hi Ed,

I didn't see a reply....

Date sent:      	Wed, 19 May 1999 21:20:33 -1000 (HST)
To:             	towertalk@contesting.com
From:           	Edward Pagaduan <ezed@aloha.net>
Subject:        	[TowerTalk] Rhombic for Field Day

> Hi Everyone,
> I don't know if this message is appropriate for this group but I am
> thinking of putting up a nonresonant rhombic antenna for field day next
> month.  I'm looking for a cheap source of noninductive 800 ohm 500 watt
> resistors or a combination.  

There are some wire wound resistors labelled as "non-inductive" 
because for all practical purposes they are non-inductive at audio 

For this application, you need composition type resistors (like used 
in dummy loads) or an array of low inductance metal film resistors 
in parallel. 

One readily available alternative is a dummy load fed through a 
12:1 step down transformer with the transformer placed right at the 
termination end of the antenna.

All of this is work and expense, and the performance of a 
conventional Rhombic is nowhere near what you might expect by 
reading the Handbook or listening to antenna stories. It has a 
narrow main lobe and generally poor efficiency at HF. The main 
advantage is bandwidth and ease of construction in a point to point 
communications system.

The bragging rights of having one, and the fun of installing one, 
often offset the fact that the materials and time would be best 
spent on other antennas. 

> acres next field day.  Have already studied the ARRL antenna book's
> rhombic antenna section. In a quandry if the power burned up in the
> terminating resistor of a nonresonant rhombic would otherwise be radiated
> in a resonant (no terminating resistor) rhombic. 

The Rhombic has some F/B (usually 3-6 dB) even when 
unterminated due to losses inherent in the system (ground losses, 
radiation losses, and conductor losses).

All terminating does, besides making the antenna frequency 
independent, is dissipate power that would otherwise be radiated 
off the backside of the antenna. It doesn't change gain any 
appreciable amount in the forward direction, it just attenuates 
backfire radiation.   

> Asia and the rest of the Pacific in that case, from Hawaii.  Or would I be
> better off with a beam for (maybe 80?) and 20 meters? We're planning to
> hold the corners up with 40 ft AB155 military surplus masts.

You don't have enough height for any beam on 80, or Rhombic. The 
The rule of thumb is the height of a Rhombic needs to be about 1/3 
of side length (considering the diamond has four sides). Below that 
height, ground losses limit gain. Gain peaks at about 3.3 
wavelengths per side, and actually deteriorates beyond about 4 wl. 

Antennas and Transmission Lines by Dr. John Kuecken of General 
Dynamics has one of the most comprehensive sections on 
Rhombics and Vees. It is out of print as an engineering book, but 
is still available as a reprint from MFJ Enterprises.   

If you are after a big score, I think a yagi for 20 and some 
directional verticals for 80 would be a better system for contest 
results. If your Field day goal is fun, there is nothing more fun than 
putting up a big uncommon antenna like a Rhombic.
73, Tom W8JI

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