Two Meter FM Antenna Question
 

I was always under the impression the 2.15 dBi and 3 dBi applied to antennas
in free space, the GP being on one side of an infinite plane at the fed end.
Indeed in real would cases in the vicinity of earth or other large (relative
to the antenna) objects the differances (particularly in FM service) become
negligable. Note too: dBi is relative to a (theoretical) isotropic radiator
(which is a mathamaticily logical standard of comparison), vs. dBd which is
relative to an ideal center fed 1/2 wave dipole in free space.


"Ed" wrote in message
. 192.196...

Every reference and factory specification for
standard grounplane and vertical dipole antennas I've seen indicates
the
standard vertical dipole has a horizontal gain of 3 dBi, and the
groundplane 2.1 dBi.



Those are pretty curious references then. Why would they dwell on the
HORIZONTAL gains of VERTICAL antennas? There must be something left
unsaid in what you are trying to express because cross polarization
would drive down sensitivities by 20 to 30 dB.

For another matter, those values you quote bear very little
resemblence to typical 2M FM operation, unless it is from the Space
Shuttle. Height above ground variations in gain easily washes over
any differences you might perceive. I can see a variation of 2dB in
just raising a groundplane from 40" off the turf to 120".


Richard,

You and I seem to be talking different languages !! :^)

My reference to horizontal gain is gain measured in the horizontal
plane...... that is, measurements are taken broadside to the vertical
antenna elements, in this case, both the vertical dipole and a
groundplane.

Gain measurements taken in any other plane in any other plane than
horizontal tend to be rather useless since most VHF mobile communications
takes place horizontally..... even distant repeaters tend to be close to
the horizon.

Cross polarization is not an issue in VHF operations since all
commercial and amateur FM operations I'm familiar with use vertical
polarization.

As far as height variations having effect on gain.... you are
talking about path gain, or system gain. I am speaking specifically of
antenna gain, which I believed to be the question of the original poster.

And as I have already pointed out, most factory specifications
for vertical dipoles and groundplane antennas are as I already listed.

Ed K7AAT

Two Meter FM Antenna Question
 

Maybe I'm missing something, who manufactures vertical vhf dipoles?


We're hams. We make our own antennas, don't we?
Two pieces of wire, and some insulators.

I had a vertical dipole for 2 meters once, a section of an old tv
antenna. One element (by definition) mounted on an insulator on a
boom. I attached the boom with two hose clamps onto my tower and
mouunted the single element out from the tower 3-4 feet. Attached a
piece of coax to the elements. Zero cost, used it for 15 years like
that.

Worked great. Wait a minute. On second thought how would I know
that? Ok let me rephrase that. "It worked." It's 2 meters, for
crying out loud - anything works. If you need gain you get a beam.

Rick K2XT

Two Meter FM Antenna Question
 

John E. Davis wrote:
. . .
Perhaps you can advise me regarding the numerical stability of the
following omni-directional, which NEC indicates has a gain of 4 and a
VSWR1.2 when fed with a 50ohm feedline at 151.75 Mhz.

The geometry consists of a 54.5 inch vertical with 4 35.625 inch
radials that are bent upward by about 11 degrees. The vertical is
made from 14 AWG wire, while the radials are 1/8 inch brazing rod.
. . .

With segment sizes of 0.05 and 0.025 lambda, the average power gain is
very close to 1. Changing the segment size to 0.0125, drops the
average power gain to 0.93, which indicates numerical instability.

Should I believe this model?

Actually, that's pretty good. It's not uncommon to see quite good models
hit around a half dB from perfect, i.e., average gain around +/- 12%
from unity. Within that range, and even when the average gain is
substantially worse, you can correct both the input resistance and gain
with the average gain figure to arrive at an accurate result. (Refer to
the NEC-2 or EZNEC manual for more information.) Poor average gain
doesn't usually indicate instability, but rather problems with the way
the assumed currents from the source overlap onto adjacent segments --
it's an excitation problem. Particularly problematic are wires of
differing diameters, different segment lengths, and with bends in the
immediate vicinity of the source. Your model has all three, so the
average gain you're seeing is really good for that model.

Roy Lewallen, W7EL

Two Meter FM Antenna Question
 

On Wed, 23 Aug 2006 03:41:41 GMT, (Rick) wrote:



Maybe I'm missing something, who manufactures vertical vhf dipoles?


We're hams. We make our own antennas, don't we?
Two pieces of wire, and some insulators.

I agree, a 2 meter dipole would take minutes to make. I was just
curious about manufacturer's published gain figures since I can't
recall ever seeing a manufactured 2 meter center-fed dipole --
apparantely there are a few out there.

bob
k5qwg


I had a vertical dipole for 2 meters once, a section of an old tv
antenna. One element (by definition) mounted on an insulator on a
boom. I attached the boom with two hose clamps onto my tower and
mouunted the single element out from the tower 3-4 feet. Attached a
piece of coax to the elements. Zero cost, used it for 15 years like
that.

Worked great. Wait a minute. On second thought how would I know
that? Ok let me rephrase that. "It worked." It's 2 meters, for
crying out loud - anything works. If you need gain you get a beam.

Rick K2XT

Two Meter FM Antenna Question
 

On Tue, 22 Aug 2006 22:59:32 -0700, Roy Lewallen
wrote:
Should I believe this model?

Actually, that's pretty good. It's not uncommon to see quite good models

The only other thing that worries me is the placement of the feedline.
I plan run it at right angles to the vertical as much as possible.
After I built my 5-element yagi for MURS, I found that the SWR was
affected by the placement of the feedline. As such, I would like to
simulate its presence with NEC-2. Any hints about how to do this?

Thanks,
--John

Two Meter FM Antenna Question
 

On 23 Aug 2006 17:34:16 GMT, (John E. Davis)
wrote:

The only other thing that worries me is the placement of the feedline.
I plan run it at right angles to the vertical as much as possible.
After I built my 5-element yagi for MURS, I found that the SWR was
affected by the placement of the feedline. As such, I would like to
simulate its presence with NEC-2. Any hints about how to do this?

Thanks,
--John

John,

Its all called out in EZNEC's help file. Look under "About
Transmission Lines"

Danny, K6MHE

Two Meter FM Antenna Question
 

On Wed, 23 Aug 2006 10:42:32 -0700, Dan Richardson
wrote:
Its all called out in EZNEC's help file. Look under "About
Transmission Lines"

Does EZNEC support linux? I do not have windows installed on my
machines--- only Debian linux. I will double check the NEC-2
docs. I also have the ARRL antenna book but I do not recall a
discussion of it there, but I will recheck that too.

Thanks,
--John

Two Meter FM Antenna Question
 

John E. Davis wrote:

Does EZNEC support linux? I do not have windows installed on my
machines--- only Debian linux. I will double check the NEC-2
docs. I also have the ARRL antenna book but I do not recall a
discussion of it there, but I will recheck that too.

Sorry, there is no EZNEC version for Linux. The last report I got was
that the available Linux Windows emulator isn't able to open the EZNEC
manual, and it has other problems with EZNEC also.

Roy Lewallen, W7EL

Two Meter FM Antenna Question
 

John E. Davis wrote:
On Wed, 23 Aug 2006 10:42:32 -0700, Dan Richardson
wrote:
Its all called out in EZNEC's help file. Look under "About
Transmission Lines"

Does EZNEC support linux? I do not have windows installed on my
machines--- only Debian linux. I will double check the NEC-2
docs. I also have the ARRL antenna book but I do not recall a
discussion of it there, but I will recheck that too.

A printable form of the EZNEC manual can be downloaded from
http://eznec.com/ez40manual.html. It's in both .rtf (rich text) and .doc
MS Word) formats, though, so you'll have to be able to read one or the
other of those.

Having no Windows system must be quite a handicap. It certainly limits
the amount of software available to use.

Roy Lewallen, W7EL

Two Meter FM Antenna Question
 

On Wed, 23 Aug 2006 12:38:25 -0700, Roy Lewallen
wrote:
Having no Windows system must be quite a handicap. It certainly limits
the amount of software available to use.

It depends upon what you want to do. I have not found it to be too
big of a limitation for what I do, and I have been using linux since
1992. I have also been an active supporter of open software for many
years (see http://www.jedsoft.org) and I have managed to code around
what I perceive as gaps.

For the antenna modeling, I use a slang script
(http://www.jedsoft.org/slang/) to search the parameter space for a
specified antenna geometry by minimizing a statistic. The slang
script runs nec and then runs a hacked version of xnecview to get the
gain and SWR as a function of frequency, and from that information
computes the statistic.

--John