[TowerTalk] Multi-Band Vertical Question

[Grant Saviers]

I have two verticals on galvanized roofs.  One is a capacity hat loaded 
40' x 3" irrigation pipe on a barn 50' x 30' for 80m elevated 20' and 
the other is a 1/4 wave Al tubing for 40m on a pole shed about 30' x 26' 
elevated 13'.  The 80m capacity hat was modeled with EZNEC and works as 
predicted - 6 x 6' x 3/8" Al tubing with a 10ga perimeter wire and 
resonance is center of 80m.

Modeling seems to be difficult for elevated solid ground planes.  I 
tried a number of wire grid configurations to simulate the roof, not 
sure that any model I tried was "real".  The other thing is that EZNEC 
is very sensitive to any ground wire from the elevated ground plane to 
"ground".  I seem to get the "best" (not sure if "real") results with a 
wire to ground in the center of the roof sheet.  This seems consistent 
with the advice to not connect elevated ground wires to ground at the 
ends.  However, I'm not sure I like the idea of the edge of the roof at 
some high RF voltage - that may be a formula for a arc/fire problem 
(which has not occurred).  (my poles are telephone poles and the barn is 
redwood)

Then there are the mystery effects of non continuous metal sheets and 
several coats of aluminized roof paint.  I do have the 4x10' sheets tied 
together with self tapping self sealing sheet metal screws and highly 
recommend that for older barn roofs.  A roof  in good shape or a 
standing seam roof likely has enough close contact/capacitive coupling 
between sheets to not need the screws.  When I erected the 40m vertical, 
I used a AIM4170 as I connected two electrically independent sections of 
galvanized roof together with jumper wires.  As theory predicts, the SWR 
went up (to 50 ohm coax) and the resonant frequency shifted (down as I 
recall but am not near my notes) as more roof became part of the ground 
plane.  Of course, if you have a soldered/brazed together copper roof 
that will likely work better than galvanized steel sheets.  I would also 
add that as long as the solid electrical interconnect points are 
separated by a small fraction of the wavelength (100:1 or better) I 
think it is unlikely that mysterious diode/rectification/IMD noise 
problems will arise from oxidized metal to metal contacts.

The web literature is very mixed about using a metal roof as a ground 
plane- from "great results" to "fuh-get-about-it".  SteppIR says "try 
it" for their vertical, which may be the best advice.  Logically, a 
conductive elevated ground plane has to be at least be the equal of 
elevated radials. 

As to operating experience, I think my 80m vertical is very competitive, 
early on I ran a bunch of ZRs through the crowd from my San Jose QTH.  I 
have an excellent radio horizon so that helps.  The 40m has been a 
relative disappointment, maybe because there are so many 40m beams out 
there to compete against.  My plan is to remove the coax ground to the 
roof and install a number of elevated wire radials.  This seems to be 
the preferred approach for verticals over salt water per DXpedition 
lore.  I also built a Reisert 1:1 coax/toroid "isolation" balun for the 
80m since the advice is to strongly decouple the feed line from the 
antenna at the feed point to reduce noise pickup and improve the 
pattern, but I did't see any significant noise improvement.

In any case, elevating the vertical and elevated radials are a proven 
win.  I'd definitely give it a try.  b.t.w. how large is LARGE?  Large 
enough for a 4 square?  That would be neat!  I've recently seen some 
covered horse arenas 80 x 200' - they could be fun!

Grant
KZ1W

My San Jose, CA ridge top QTH is for sale - 50 acres, awesome views, 
some antennas in place, three towers on the ground  see

http://www.coldwellbanker.com/property?action=list&freeTextAddress=95140&countryId=1#modalDetail


Gary "Joe" Mayfield wrote:
> Has anyone looked at installing one of a LARGE metal roof, and just using
> the roof as the ground plane?
>
> 73,
> Joe kk0sd
>
>
>