[SCCC] Roof verticals

Thu Nov 26 13:54:34 PST 2009

>From KD6HTN's QRZ page I see the antenna is a GAP Eagle, which uses 3 rigid
80" counterpoises. It doesn't look like a vertical dipole design. I'm not
certain of how it's put together but a roof location should do better with
tuned wires rather than a ground plane. A couple of 1/4 wavelength wires on
the roof for each contest band might have good results. 4 notched twinleads
a la the Butternut design (see www.Bencher.com) would cover 10-15-20-40.

On the other hand, I've operated at the California Speedway with various
verticals duct-taped to a railing attached to thousands of tons of metal
bleachers, and using no radials. They didn't tune well, but they were good
for a few hundred CW contacts without much effort. Just put it up and tune
it close enough and work 'em.

John kq6es

-----Original Message-----
From: sccc-bounces at contesting.com [mailto:sccc-bounces at contesting.com] On
Behalf Of Michael Tope
Sent: Thursday, November 26, 2009 1:22 PM
To: Kate Hutton
Cc: sccc at contesting.com
Subject: Re: [SCCC] ARRL Sweepstakes SSB

Kate Hutton wrote:

>This antenna is pretty strange.  15 m is completely unusable; the SWR is
>wacko.  10 m is fine (or at least tunable), 12 m is fine, 17 m is fine, 20
m
>is fine.  No-one has been able to satisfactorily explain to me why this is,
>except that "there is other metal in the area" (such as dog runs in the
back
>yard).
>
>  
>

I can't really say what may be going on with your antenna as I don't 
have any experience with that design and what mechanism they use to 
achieve multiband coverage. If you haven't checked it out already, 
"Towertalk" is a good resource for this sort of thing:

http://www.contesting.com/FAQ/towertalk

The list has a pretty large subscription base so odds are good that 
there are a few GAP antenna aficionados on the list that can help you. 
Courtney Duncan N5BF from at JPL has a GAP antenna at his home QTH in La 
Canada. He is a pretty sharp guy, so he may be able to provide some 
useful input as well (I don't recall what model GAP vertical he has).

The notion that "there is other metal in the area" can explain poor 
performance, but to totally mess up VSWR the way you describe the 
offending metal typically has to be pretty close to the antenna radiator 
(<= 1/4 wavelength at the operating frequency) and the dimensions have 
to be on the order of the antenna dimensions and in the same plane as 
the radiator (a 1/2 wavelength long aluminum rain gutter probably won't 
couple much if it is perpendicular to the antenna radiator). This may be 
a problem with the antenna itself. I had a problem with 12 meter tuning 
on a Cushcraft R7000 recently. Jeff KE6L clued me into the fact that I 
needed to tweak the inductor in the 12 meter trap a little to get proper 
tuning. Sure enough a little extra inductance pulled the VSWR dip down 
from 26 MHz to 24.9. The adjustment was very sensitive.

>>I've been running some comparison tests between a Cushcraft R7000 with the
>>base at about 8' and inverted-Vs at 35' (80/40/20 meter inverted Vs with a
>>common apex). On close-in stuff (out to say 500 miles), the inverted-Vs
>>clobber the R7000. On longer distance stuff (> 1000 miles) the R7000 is
>>often equal and occassionally better in terms of absolute S-meter reading,
>>but almost always more noisy than the inverted-Vs. Depending on the time
of
>>day, that might explain why you did better on 20 than 40 (if 40 is
primarly
>>supporting short skip a vertical can put you at a big disadvantage
compared
>>to a horizontal antenna).
>>
>>
>>    
>>
>Comparing similar skip distances, eastern & midwestern US, my impression is
>that fewer stations hear me on 40 m than on 20 m.  I had already noticed in
>my CW experiences, both far & near, on 20 m stations can copy me & I can
>barely copy them.  On 40 m it's the other way around.
>
>Kate
>
>  
>
Kate, your observations suggest that your antenna efficiency may be poor 
on 40 meters. On HF frequencies (and especially lower HF frequencies), 
poor antenna efficiency doesn't hurt you much on receive. You can easily 
see this if your rig has a receiver step attenuator. In most cases 
(unless you are in an extremely quiet rural location), adding 10 or 20dB 
front-end attenuation in on 40 meters doesn't impair you ability to copy 
signals. This is because your ability to copy is not limited by absolute 
signal level, but rather by the ratio of received signal level to the 
strength of the received man-made noise. If you add attenuation, both 
the signal and the noise go down by the same amount - the 
signal-to-noise ratio (SNR) is unchanged. A simple test you can do to 
determine if you have sufficient antenna efficiency for receiving is to 
tune to a clear frequency which is representative of your background 
noise level and then disconnect the antenna from the radio. If the 
background noise drops when you disconnect the antenna, then your 
antenna is efficient enough for receiving on that band.

On transmit it is obviously a different story. When you lower your 
transmit power, your signal level at distant points goes down, but the 
noise level at those distant points does not. In effect, the 
signal-to-noise ratio (SNR) of your signal at those distant points is 
reduced. Transmitting into an antenna with poor efficiency is equivalent 
to lowering your transmit power (i.e. transmitting 100 watts into an 
antenna with 10% efficiency is equivalent to transmitting ~11 watts into 
an antenna with 90% efficiency). Thus poor antenna efficiency could 
explain why you hear signals just fine on 40 meters, but those stations 
can't hear you very well.

With regard to your situation on 20 meters, the fact that stations can 
hear you even when you can barely hear them suggests that your antenna 
efficiency is probably better on 20 meters than on 40. It also suggests 
that your man-made noise floor on 20 meters may be higher than the 
man-made noise floors of those stations that you are working. The claim 
is often made that verticals make for noisy receiving antennas. This is 
true in locations where the noise is locally generated and propagated by 
groundwave. Groundwave propagated noise on LF, MW, and HF frequencies is 
always vertically polarized. In a quiet rural location where the bulk of 
the noise arrives from distant locations via skywave, the polarization 
of the noise is random so in those cases the vertical can be just as 
quiet as a horizontal antenna for receiving. Since you live down here in 
the LA Basin, odds are good that your noise floor is dominated by 
locally generated sources that propagate via vertical polarization. Thus 
your receiver noise floor on 20 meters may be higher than average. I 
know that is the case with my vertical antennas here in Tujunga. Twenty 
meters is terribly noisy here when I am listening on my trapped vertical.

I agree with Cliff, K3LL. If you have room for a horizontal antenna on 
your property, I think you will be pleasantly surprised at the results 
you will get, especially for shorter distances. It is also great fun 
when you have more than one antenna to switch between and compare signal 
strengths. If you do put up a horizontal antenna and feed it with coax, 
make sure you use a good common mode choke at the feedpoint. A good 
common-mode feedline choke will help reject vertically polarized noise 
that tends to get coupled in from the vertical sections of the coax feed 
to a horizontally polarized antenna. Jim Brown K9YC has a great treatise 
on building these common-mode feedline chokes on his website:

http://www.audiosystemsgroup.com/NCDXACoaxChokesPPT.pdf

Here is some additional inspirational information for city dwellers 
looking to make a go of it on HF:

http://www.audiosystemsgroup.com/K9YC/k9ycant.htm

Hope this is helpful.

73, Mike W4EF...............

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