[Elecraft] lo-noise verticals

[Eric Manning]

Chas
I have a centre-fed, shortened [by centre loading coils and 
capacitance stubs on the ends],  halfwave vertical [ a Force 12 Sigma 
5  centre-fed dipole stood on its ear] on the stern of my boat on 
salt water,  and I used a bobtail [2 quarter-wave verticals fed 
in-phase using a full-wave centre-fed dipole as the phasing line] 
this summer on the edge of beautiful Lake Huron, all on 20m.


Both are noticeably lower noise than my horizontal  dipoles. [I 
converted a 7 MHz dipole into the 14 MHz bobtail by hanging 
quarterwave wires from its ends, so I had a dipole to compare it to.]

K4XX the bobtail guy [July 02 QST] says that a lot of  noise arrives 
at high angles of incidence which is why the bobtail is  lo-noise. 
The bobtail is lo-angle and the Sigma 5 is too - it can be as low as 
10 degrees over salt water according to the designer, which mine is 
[being 3 feet above the Pacific Ocean!]

Also, some of the noise has got to  be horizontally polarized.

I had very gd DX results this summer from both with the K2 
-kazakhstan, croatia & malta.

[speech]
Anybody interested in portable or maritime mobile verticals should 
get or build a centre-fed   halfwave vertical, full length or 
shortened, and forget all that stuff about quarter-wave verticals 
using  insulated backstays,  and trying to get an rf ground  with the 
keel IMO. The centre-fed vertical dipole removes much [all?] of the 
need to get a near-perfect rf ground. It does not rely on the ground 
to provide one half of the halfwave dipole as does the traditional 
quarterwave vertical at ground level.

As for fixed use, I have had good luck with quarterwave verticals at 
least a halfwave  in the air with several radials, also up in the 
air, for rf ground, but terrible results from quarterwave verticals 
with their bases on the ground. I just couldn't get a good enough rf 
ground from the earth.
[end speech]

I'm going to use the Sigma 5 portable S2 if the Bangladesh Telecomms 
Regulatory Commission will issue a letter of permission to operate 
[fingers crossed, touch wood]. If so, we'll see how it does with the 
K2 for land portable.

eric on the other coast


Message: 13
Date: Sat, 28 Sep 2002 14:11:21 -0400
To: elecraft@mailman.qth.net
From: Charles Greene <W1CG@QSL.net>
Subject: [Elecraft] Portable antenna

Hi,

A week or so ago, Wayne was asking about a low noise and I recommended a
vertical.  Here's my latest finding:

I made up a 20 meter vertical for portable use and installed it on my sea
wall for test. It is full size, and I used some junk 1" aluminum tubing I
had in my attic. It has two wire radials elevated 1' above the ground. I
made the radiator slightly short and the radials about 2' long to bring up
the feed impedance to 50 ohms and also the impedance of the radials so one
of them does not hog the current which it is inclined to do if all radials
are at zero ohms.  The first contact I made was day before yesterday, and I
got a S9+20 report from the Fiji islands, running 30 watts on PSK31. Then I
ran some tests on it. It is located about 50' farther away from the house
than my 6BTV and G5RV. Using Spectrogram, I determined the noise floor was
10 dB (voltage) lower than the 6BTV and 16 DB lower than the G5RV. The
received signals were about the same as the other two, with a slight edge,
1 to 2 DB stronger. On comparative reports I have received on transmit, it
is running "slightly" to 1 S unit more than the other two. On DX, it is at
the edge and 8' above sea level at high tide of Narragansett Bay, 4 miles
wide at this point. On the other two antenna, my 6BTV has 22 radials of
480' of wire, and the G5RV is 30 ft high and about 50' higher than the sea
level of the bay at high tide.. Both seem to work well. I knew the G5 was
noisier than the 6BTV, but that usually does not come into play unless the
signals are so weak they are in the noise level.
I really can't explain why the vertical is so low noise (my house isn't
that noisy), and I would have guessed it would have performed a little less
well than the doublet and the vertical antenna with a fair number of
radials. I can understand why it is better on the long haul stuff, as it
has nothing between it and the frezonal zone a mile or two away. I would
appreciate any comments.




73, Chas, W1CG
K2 #462

--__--__--
-- 
Eric Manning   VA7DZ VE3DPV
K2 #2561 "Pauline"

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