[TowerTalk] QUAD FEEDING

[VR2BrettGraham]

"bobt" asked:

>I'm ready to put a Lightning Bolt 2 element, 20/15/10m quad up at 80ft. I 
>want to do this right the first time, and get the best reasonable 
>performance from the antenna. Lightning Bolt supplies a 2:1 balun for a 
>common feedpoint. The owner of the company sez this is the best way to 
>feed the antenna for max performance, anyone who sez otherwise is wrong. 
>But...he can't supply figures or pattern pics. Others favor the gamma 
>match method, but still I haven't seen any concrete evidence that this is 
>the best way. (believe me, I've scoured the web) Still others say a common 
>feed point (feed line)is just no good; the only way to go is seperate 
>feedlines for each band. Still no facts or figures to back up this claim. 
>I am primarily concerned with maximum forward gain and a fairly good 
>pattern. I won't quibble over .5-1.0 db or so. If it makes any difference, 
>I am also interested in integrating a 3 element 6 meter quad on the same 
>boom. (seperate feed) and/or mounting a loaded 40 mete
>  r dipole along the boom of the quad. So, who has the facts? Please, no 
> educated guesses or second hand anectdotes. Thanks.

Have a look at W4RNL's analysis of feeding quads (www.cebik.com).

I've put up four LBQs, two of them I regularly used.

I recently had a chance to use one that has split feeders &
proper choking of the feeders at the feedpoints (mine has
been replaced with a Force-12 C-3E/A).

Must have been the right time for the height of the antenna & the
arrival angle of the signal, but in DFing a CODAR (sea surface
radar) signal that has taken over our 12m band in SE Asia, I was
able to see a lovely, symmetrical main lobe centered between two
well defined side nulls & a clean rear pattern with another
reasonable null.

Of the four LBQs, the next two went straight to separate feeders
after the first was converted.  The fourth wasn't really used so much
& was left with a combined feed.  The original combined feed was
used on the two with spit feeders as the 17-15-12m drivers are still
combined (20 & 10 being fed on their own), but I've destroyed two
of supplied combined feeds in RTTY contests at <500 watts output.
I believe the potting compound used to make them doesn't shed the
heat too well & even at low power, they might not be up to a higher
duty cycle.

The spacing on 10m is a bit wide & becomes obvious by looking at
the match when fed on its own.

The LBQ is a great antenna & the maker is a great guy (he even
replaced all the wire holders for one free of charge after I found
the worm screws on the jubilee clips weren't stainless), but to
suggest the combined feed is the best way to go is a bit naff.

Oh, yes - two of the four had loops for 3-el on 6m added.  If one
has a problem with the driver sticking down near the mast, do
not use a yagi driver with gamma match as the boom is close
to reasonance & you end up with something more like a turnstile.

Have no suggestions about how to add 40, though if you look
around there are a number of possible ideas others have used,
though the methods that require connecting other than at the
end of the loop may be problematic because of the wire used
(like those that add loading to center of vertical sides of loop
through a series trap).

If I were to do it now, I would strap fiberglass poles to the ends
of the boom & make a shunt-fed rotary doublet out of the boom
with wire extensions.  I would not do this with the 2-inch OD
boom & 40 or no 40, would go for the 3-inch OD boom if you
have not yet purchased the LBQ should you erect it with slide-
boom-to-one-end-bolt-on-element-then-slide-to-opposite-end-&-
bolt-on-other-element technique (one element on end of 8-foot
boom sags too much for my liking, though pucker factor starts
with 20 dB correction factor if that boom is sticking out from free
standing tower a meter or so away from edge of roof 10 floors
above street level ;^).

73, VR2BrettGraham