I am NOT taking sides on this thread, but reporting my twenty five year
experience with NVIS and vertical antennas at my location.
My conditions are a rural subdivision one acre lot, with several feet of
glacial moraine black soil over water bearing clay at 3 to 5 feet. The NVIS
antenna is an all-band doublet some 130 feet overall in a inverted VEE
configuration, apex at 12' fed with 450 Ohm window line. The vertical is an
old Butternut HV-2 (80-40) top loaded with four 12 foot wire umbrella to
cord guys. The 160m base loading kit is usually in place. The vertical is
mounted on a 12 foot mast which was was "water hose driven" 10 feet into the
ground. Twenty four radials are made from surplus telephone central office
jumper wire (# 20 -# 22 ?), each approximately 65 feet in length, laid on
top of close cut grass over the septic leach field. The feed line is direct
burial RG-8 laid on the ground.
The NVIS performs as expected out to about 500 miles. The vertical has
performed better than expected for DX, consistently getting through on
gray-line, and breaking pile-ups. Local DXers returned from DX-peditions in
report the signals from my vertical is usually stronger than those from
stations in the region.
I do not use low dipoles other than the NVIS because of my focus on HF DX.
I do have a sloper but find the vertical produces a stronger signal for DX.
I have used a low wire Beverage style RX antenna with excellent results.
73 es DX,
Gary - AB9M
-----Original Message-----
From: Steve Hunt
Sent: Wednesday, January 05, 2011 4:38 PM
To: Discussion of Ten-Tec Equipment
Subject: Re: [TenTec] New and Improved Terminology (NVIS origins)
There are two distinct and different issues here. Take a look at how the
ARRL Antenna Book tackles them quite separately.
1. Antenna efficiency: directly affected by the ground conductivity in
the Reactive Near Field. This is the parameter we are usually improving
by laying down a good ground radial system
2. Elevation angle pattern: determined by soil characteristics in the
Far Field where ground reflections are taking place. Do some simple
geometry and you'll see that at really low elevation angles - 5 degrees
say - the ground reflections from an 80m quarter-wave vertical will
extend out to at least 750ft from the base of the vertical. A typical
ground radial system will do nothing about the conductivity there.
You can check this using EZNECs ability to have two different ground
media. Set up Media 1 as salt water extending for about 50ft around the
vertical, and Media 2 as rocky soil beyond. Look at the elevation
patterns - they will be those which pertain to poor ground. Now
gradually extend Media 1 outwards until you get the elevation pattern to
match that over salt water - that will tell you how far out the
reflections are taking place and how extensive your radial system would
need to be.
Please note - I'm not saying a good ground system doesn't help. I'm
saying that the usual ground radial system predominantly affects antenna
efficiency and not elevation pattern.
Please also note that nothing I have said conflicts with the Exam
Question answers.
73,
Steve G3TXQ
On 05/01/2011 21:44, Richards wrote:
Er... ah... No joke. Do you doubt the proposition that a
good ground plane lowers radiation take off angle? If so,
I would be curious as to your reasons. (Seriously, and with
no intention to flame or cause an argument, as I have
made quite an investment in time and resources in vertical
developing a reasonably decent vertical antenna system
for my small, suburban back yard. Any info would be
greatly appreciated. )
FYI ---
------ Extra Class Exam Questions of interest -----
Question E9A12 - and the answer is that the efficiency
of a quarter wave grounded vertical antenna can be improved by
installing a good radial system.
Question E9A13 - answer is - soil conductivity is the most important
factor in determining ground losses for a ground-mounted vertical
antenna operating in the 3-30 MHz range.
Question E9C13 - answer is - When a vertically polarized antenna
is mounted over seawater versus rocky ground, the far-field
elevation pattern low-angle radiation increases.
Question E9C17 - answer - The main effect of placing a vertical
antenna over an imperfect ground is that it reduces low-angle
radiation.
Also, on the audio study guide, Gordo makes some stray comments
about using 3 inch copper strap for radials on his roof, and
mentions improving the ground field increases low angle
radiation - he goes on to say it does not increase signal strength,
of course, but only that it increases the amount of signal that
has low take off angle.
This information is consistent with all that I have read on verticals
in the ARRL Antenna Handbook, and I did LOTS of research before
installing a large vertical monopole in the back yard.
A huge ground pane does lower take off angle (as NEC modeling
shows) and also improves antenna efficiency -- I stopped at 65 radials
but I wish had installed even more just to be sure. Also, my back yard
soil is very conductive and remains moist even through the summer, so
the soil, itself, helps me considerably. Rob Sherwood and I exchanged
some nice email at the time I was doing this homework, and I believe
he lives over a more dry, rocky soil, and that is much harder to work
over. I also corresponded with the infamous Rudy Severns N6LF
and his findings are consistent with this conclusion. NEC modeling
produces consistent results.
N'est ce pas? Happy trails OM.
=============== JHR ============================
I had
On 1/5/2011 1:01 PM, Steve Hunt wrote:
I presume the "wink" indicates that is a joke !
The only thing that would improve the elevation pattern would be
improved ground conditions in the Fresnel Zone where the ground
reflections are taking place. That would take some awfully long radials
- certainly well beyond my property boundary - and an awful lot of them
to achieve a copper density that would affect the conductivity so far
out.
73,
Steve G3TXQ
====================================================
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|