From: rdewan@casbah.acns.nwu.edu (Rajiv Dewan)
Subject: Re: HF Loop Antenna
Keywords: How to build?
Organization: Northwestern University, Evanston Illinois.
Date: Fri, 20 Nov 1992 21:56:20 GMT

In article <1992Nov20.210346.14965@cs.yale.edu> oswood@rtnmr.chem.yale.edu (Mark Oswood) writes:
>
>I am thinking about building a loop antenna similar to the AEA isoloop.  
>It seems like a great antenna design for limited space without losing too
>much efficiency.  However, I can't afford the $300 AEA charges, and it 
>doesn't seem impossible for me to build.  I have found an article in 

Check out the ARRL antenna book.  It is a great reference for this antenna.
Has lots of design data.  There are two flavors of the small transmitting
loop antenna:
i) Lew McCoy's Army loop: allegedly used in the Korean war to great effect
ii) Ted Hart's (W5QJR) loop.

The differences are in the feed point and capacitor location.

There is some information in Moxon's "HF Antenna for All Locations", 
published by RSGB.  The section in the ARRL antenna book on small 
transmitting loops was written by Ted Hart.

>June 1986 QST which is helpful, but I am wondering about what capacitor 
>to use.  For those unfamiliar with the design, this is a loop of 
>conductor approximately 1 meter in diameter with a variable capacitor in 
>series with the loop.  The loop is coupled to the feedline through a 
>gamma match or a smaller loop.  Apparently I need a variable in the 
>range of 5-200 pF which is able to withstand at least 10,000V 
>(assuming I will limit my transmissions to ~100W).

And preferably, is of the split stator design.  These are much better
for transmitting applications with heavier RF currents as they do not
have a wiper for the rotor.  

BTW, if you are considering a single band version, consider making a 
full size (1/4 wavelength circumfrence) version.  The Q of such an
antenna will permit operation over a good portion of the band without
requiring a variable capacitor.  A fixed value  high voltage capacitor
can be assembled by soldering alternate pieces of double sided unetched
PC board.  

>
>1.  Can anyone suggest a source for such a capacitor?
>

Ted Hart, W5QJR, sells them.  His address may be found in the ARRl Antenna book
or use the 92 CBA.

>2.  Has anyone actually built such a loop, and do you have any suggestions?
>

Yes, I built one for learning about such antennas.  I built a full size
one (still a small transmitting antenna) for the 10m band.  No question 
that this is a compromise antenna.  It was 2 to 5 S units worse than my 
usual loop antenna: a 80m full wave loop.  I guess the small loop takes up 
somewhat less space. :)

A few words about the AEA version:  This antenna is deceptively simple.  In
reality it is tough to make one that works well.  The reason is the low
impedance: often the radiation resistance is a small fraction of an Ohm.
It is non-trivial to make such antennas efficient.  It requires large 
skin areas for the conductor, as few joints as possible and that all 
joints be completely bonded electrically.  This is much tougher to attain 
at higher HF frequencies.  Mechanical connections just do not suffice.

I do think that $300 is a bit much.

When a small loop is operated at the lower end of its frequency range, its
Q may be so high that the bandwidth may be less than that required  for SSB.
Another reason for narrow bandwidth modes. :)

>I hope this is the right group for technical questions--from reading one would
>think this was alt.cw.flame. :-} 

It is sad isn't it?

Rajiv                                 
aa9ch    
Address: r-dewan@nwu.edu
Phone:   None. Only CW.
And occasionally AMTOR.
Newsgroups: rec.radio.amateur.misc