[Elecraft] Re: Elecraft digest, Vol 1 #123 - G5RV and baluns

[John Crux]

The original G5RV was NOT a multiband antenna, so a balun was not 
really needed.  In those days the nearest you could get to ladderline 
was that ghastly 300 ohm ribbon feeder intended for TV use.  Its 
worth remembering that G5RV designed the antenna before the solid 
state era .... A tube final stage with a pi-section tank circuit 
could be made to load into things that cause a modern solid state 
final to barf. 
My first (and only) ham radio job was in a small, long forgotten firm 
that made tube AM/CW rigs custom designed by G5RV, all with 
pi-section PA tank circuits. That was nearly 50 years ago, but I 
still remember meeting Mr Varney there. (Gosh am I THAT old ?)  

It seems we have adopted his venerable design for things which were 
never intended.  But we have an advantage ... modern ladderline is a 
darned sight better than the old 300 ohm TV ribbon.  And having got 
an efficient balanced antenna setup, why not preserve that balance 
and at the same time give the KAT2 an unbalanced line to load into ?? 
Answer - use lots of ladderline, not too much coax, and a balun, not 
for matching, but just to interface the ladderline nicely to coax. 
The total system SWR will be mostly lousy, but who cares as long as 
we can work at least 7 bands with one antenna, without excessive 
losses..

My "G5RV" is a bit short - its a 40m dipole, thanks to a major error 
of judgement when I threw it up.  I use around 60 feet of ladderline 
to a 4:1 antenna tuner balun and then 20 feet of coax. to the KAT2. 
The setup works even on 20m where the 40m dipole presents a high 
impedance (two half waves in phase.)  This morning ZK1HCC on N Cook 
had no trouble copying me - his 5 watt FT817 was 569 here. The balun 
seems to introduce no significant loss and the KAT2 copes wonderfully 
well, on all bands 10-40. It will load on 80, but the antenna is 
really far too short to be much good. It is very wrong for 6m but I 
only need Oceania for WAC with the K2 and a TenTec 6m transverter. 
 
But after reading W2FMI's many articles on the subject of baluns, it 
seems a good idea NOT to skimp on the size of the balun core. In a 
multiband, mismatched cheap and nasty setup like mine, the balun is 
probably the biggest potential source of loss. 

I recently followed one of the W2FMI designs to make a balun for a 
friend who runs 10w into a G5RV fed with 400 ohm ladderline. . I used 
a T200-2 two inch powdered iron core, but as predicted by W2FMI, it 
does not have really enough inductance for 80m operation of the G5RV. 
I tried it with both my K2 and an LDG AT-11 auto-atu, but its no 
good.  I'm now going to try a hefty 3 inch OD core.

If you must use a smaller core then it probably needs to be ferrite, 
with a permeability of around 125, to get adequate inductance. But 
personally I'd not use one less than 2 inch OD.  A single 2.4 inch 
ferrite core should give a 4:1 balun good for around 100 watts (at 
least) from 160-6m if W2FMI is right.  I have tried a 7/8 inch 
ferrite core, but it was a bit of an attenuator. Over-kill is 
definitely preferable to minimal size/weight. I'd rather backpack a 
roll of ladderline and a large balun than a roll of RG-8 coax.

John G3JAG