On Mon, 24 Jun 2013 11:04:01 -0700, (Dave Platt)
wrote:

In article ,
Jeff Liebermann wrote:

I overlooked the mobile operation from a car. Of course, when the
antenna is mounted on a metal surface, you don't need radials..

If it's a metal car roof, you don't need radials. Unfortunately, I've
had to deal with verhicles that have a fiberglass roof. Aluminum duct
tape ground plane (on the inside) to the rescue.

That will help but not entirely resolve the situation.

What I have heard, is that the theoretical gain advantage of a
5/8-wavelength monopole over a 1/4-wave monopole, is dependent on the
antenna being operated over a fairly large groundplane (one which
reaches out several wavelengths from the feedpoint). A simple set of
ground-radial "tapes" won't be big or extensive enough... and,
actually, neither will be the typical vehicle roof (at VHF wavelengths
at least).

Umm... if that were true, then 5/8 wave base station antennas, which
all have 1/4 wave ground radials, shouldn't work or require extra long
ground radials. Here's a stacked dual 5/8 wave 6 meter ground plane
antenna:
http://802.11junk.com/jeffl/antennas/gnd-plane-05/index.html
(Note that the characteristic impedance is about 125 ohms and that the
necessary matching xformer is not shown). Plenty more 5/8 wave base
stations antennas with 1/4 wave radials found with Google images:
https://www.google.com/search?q=5/8+wave+base+station+antenna&tbm=isch


However, I will confess that the aluminum duct tape ground plane was
easy. The fiberglass roof had stiffener ridges, projecting lamps,
other antennas, and metal stiffeners. Wiggling the tape around these
was not easy. I finally gave up, dropped the entire headline, removed
all the other hardware, and covered most of the roof with the foil
tape. It wasn't an electrical issue, but rather that we didn't have
time to do much experimenting. There was no reason to minimize the
use of the aluminum duct tape, so I just plastered it onto the
underside wherever possible. I therefore did NOT determine if 1/4
wave radials were adequate. Also, no swept response with an antenna
analyzer... just a VSWR check.

According to these sources, in the absence of a good groundplane, the
5/8-wave monopole tends to "squint" - its highest-gain lobes are not
towards the horizon but aim upwards somewhat. Gain towards the
horizon may be *less* than a quarter-wave monopole on the same vehicle
mount.

I would think it would be the other way around. Large ground planes
are more reflective causing more of the RF to go towards the sky. I'll
need to run an NEC2 model to be sure. However, at VHF, I don't think
it's a problem. The vertical radiation angle of a 5/8 wave antenna is
sufficiently wide that a small change in takeoff angle isn't going to
make much of a difference in coverage.

End fed collinear antennas, with or without a ground plane, usually
have a non-zero takeoff angle. If you want the major lobe to point to
the horizon (i.e. zero takeoff angle), the antenna should be center
fed, which is not going to happen on a mobile antenna. However,
that's not necessarily a good thing, as such a wide vertical radiation
angle antenna, that is so close to the ground, is going to send much
of the RF into the absorbent ground. Better to have some uptilt and
hope that some of it goes in the right direction.

So, the theoretical gain advantage of a 5/8-wave vehicle antenna may
not work out in practice. Testing would be required to see if there's
actually an advantage, or whether a "high gain" antenna of this sort
is actually a loss in practice because the gain is aimed in the wrong
directions.

That doesn't sound like it would be easy to test on a vehicle. I
think a computer model might be easier and probably more interesting.
(No, I'm not volunteering to do one).

And, I agree that for many vehicle mounting situations, a "ground
independent" antenna such as an end-fed half-wave may be the best bet.

Yep. I have quite a bit of experience with 1/2 wave antennas on
fiberglass vessels. They work just fine. However, there's an
additional problem on marine applications which limits antennas to
fairly low gains. If the gain is too high, and the vessel rocks and
rolls with the waves, the narrow radiation angle could easily send the
signal into the sky or into the water, instead of towards the horizon.
There's a similar problem in vehicles going up and down hills, but is
less serious. Fortunately, for VHF, it's not too horrible.

I believe you can get these in a shortened form (with distributed or
lumped inductive loading in the center of the radiator) to keep the
height within reason... but going for a full-length end-fed radiator
would give you somewhat better gain and efficiency, if it's safe to
install on the vehicle.

Yep.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

It would have helped - had the OP posted the model number for the antenna and a description of what he wanted to do with it.

A mobile vertical antenna has no gain - gain is only achieved when you have gain in one direction and rejection in one or more directions.

The only measurable gain would be gain as compared to a dipole or gain over isotropic.

1/4 wave antenna's can sometimes produce a better signal locally, because the radiation pattern is spread out over a larger area.

It will give no distance gain - just local reception.]

Because it is all one radio - the reception length does not matter, just that it is resonant at X mhz - transmit.

Public service here all uses Larsen antenna's - especially the PA State Police, and they have very deep pockets.

Shakespeare also makes a decent mobile antenna.

You need a antenna analyzer or a Dip Meter to set to resonance.

The only thing the SWR meter can do is tell you what is happening in the feed line.

On Wed, 26 Jun 2013 13:16:39 +0100, Channel Jumper
wrote:

A mobile vertical antenna has no gain - gain is only achieved when you
have gain in one direction and rejection in one or more directions.

Wrong. Gain on a mobile antenna can be achieved without
directionality. Just reduce the vertical radiation angle, which puts
more RF towards the horizon, and less RF towards the sky and the
ground.

The only measurable gain would be gain as compared to a dipole or gain
over isotropic.

Yep. That's the way gain is normally measured. dBd or dBi.

1/4 wave antenna's can sometimes produce a better signal locally,
because the radiation pattern is spread out over a larger area.

I've seen that. However, it's usually the result of misusing a "gain"
type antenna, such as a dual band 5/8 ham antenna being used on marine
or commercial frequencies, or a 5/8 commercial antenna, being used on
ham frequencies. Lots of ways to do it wrong. Where a 1/4 wave
antenna really shines is when one needs to cover a wide range of VHF
frequencies, from aircraft to marine.

It will give no distance gain - just local reception.]

Ummm... the range depends more on the terrain than on the antenna.

Because it is all one radio - the reception length does not matter, just
that it is resonant at X mhz - transmit.

VSWR is highly over-rated. The only real reason to keep VSWR low is
that high VSWR will cause the transmitter to protect itself and partly
shut down.

Try this experiment. Take a piece of sheet metal (or aluminum foil
covered cardboard) to act as a ground plane. Insert and SO-239
connector in the middle. Add a length of moderately stiff electrical
wire to the SO-239 that is longer than 1/4 wave at the weather
frequency (162.xxxx). Find a receiver that will measure the actual
receive signal strength. An all mode or AM (not FM) receiver will
work nicely. Extra credit for using a service monitor. Make a
measurement and start cutting the length of the antenna in roughly
1/2" intervals. Measure the receive signal strength.

What I've found when I've done this, is that the antenna gain, which
is what the receive signal level indicates, doesn't change very much
until you get down to about 1/8th wavelength. I modeled this test
using 4NEC2 and found the same thing.

Now, if you believe that the tx and rx performance of an antenna are
identical, this would suggest that you could make the antenna almost
any length, and still have adequate gain and function if you could fix
the VSWR.

Public service here all uses Larsen antenna's - especially the PA State
Police, and they have very deep pockets.

Obviously, the more expensive the antenna, the better it works.

Shakespeare also makes a decent mobile antenna.

They mostly make marine and military antennas. Their commercial
antennas are overpriced versions of the antennas that they sell to the
military. They're very well built, rugged, but not cheap.
http://shakespeare-military.com

You need a antenna analyzer or a Dip Meter to set to resonance.

Have you ever tried to resonate a 1/4 wave antenna with either of
those? You'll find that it's affected by the position and location of
just about everything within about a 20 ft radius. I run a sweep
generator, directional coupler, detector, and scope combination to
test antennas, but no way would I ever use that to tune the antenna.
Just getting near the antenna ruins the display. Incidentally, for
complex antennas, such as a dual band J-pole, minimum VSWR isn't
always at resonance.

The only thing the SWR meter can do is tell you what is happening in the
feed line.

Wrong. A VSWR meter reading is affected by the xmitter output
impedance, feed line impedance to the VSWR meter, characteristic
impedance of the coax cables(s), feed line impedance after the VSWR
meter, and of course, the antenna impedance. That's actually a
problem because a VSWR meter is affected by literally everything.

Drivel: I run mostly 75 ohm systems (because the coax is cheap and
easy and has less loss). I had to build my own 75 ohm directional
coupler in order to get accurate VSWR measurements. (Yes, Bird makes
a 75 ohm wattmeter 4307, but I don't want to spend the money).

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

And so Jeff speaks.

Jeff - the radiation pattern of a 1/4 wave antenna pancakes compared to a longer antenna, but you cannot use weather radio as a barometer to measure how efficient a antenna works.

Weather radio is broadcast in such a way that a signal is available to most people within a 40 mile radius circle from each transmitter - at least in Western Pennsylvania. The signals here - about 350 watts at the transmitter - is enough that a simple table top radio or a cheap bubble pack GMRS radio will usually receive it - even with a 4 inch antenna.

However - when working any type of DX - you best better bring your A game or go home. You are not going to net much with a simple 1/4 wave antenna on VHF frequencies when the bands are not wide open.

This man wants to replace his mobile antenna with something better, but does not want to take the advice of his radio technician - because he thinks the guy is ripping him off.

The bottom line is - most people involved in communications doesn't just start selling radios without any type of formal education.
Even if the only education the person received was from the Military, it is usually based on sound practices and principals.

I don't usually deal with anyone that carries their money in a snapper purse.

If someone wants to tell me how to do something that I have been doing for 40 years - I just walk away.
I laugh at these so called roadside CB radio shops that sells all this garbage to these CB radio guys, including their peek n tunes - which does nothing except reduce the life of the radio.

Good Catch Jeff, and thanks for straightening thins out -- it's amazing
some of the things we think we know about antennas and antenna measurement
until someone corrects us!
Furthermore, it's difficult for someone who does not fully understand the
concepts to receive
misinformation!

Irv VE6BP

"Jeff Liebermann" wrote in message
...
On Wed, 26 Jun 2013 13:16:39 +0100, Channel Jumper
wrote:

A mobile vertical antenna has no gain - gain is only achieved when you
have gain in one direction and rejection in one or more directions.

Wrong. Gain on a mobile antenna can be achieved without
directionality. Just reduce the vertical radiation angle, which puts
more RF towards the horizon, and less RF towards the sky and the
ground.

The only measurable gain would be gain as compared to a dipole or gain
over isotropic.

Yep. That's the way gain is normally measured. dBd or dBi.

1/4 wave antenna's can sometimes produce a better signal locally,
because the radiation pattern is spread out over a larger area.

I've seen that. However, it's usually the result of misusing a "gain"
type antenna, such as a dual band 5/8 ham antenna being used on marine
or commercial frequencies, or a 5/8 commercial antenna, being used on
ham frequencies. Lots of ways to do it wrong. Where a 1/4 wave
antenna really shines is when one needs to cover a wide range of VHF
frequencies, from aircraft to marine.

It will give no distance gain - just local reception.]

Ummm... the range depends more on the terrain than on the antenna.

Because it is all one radio - the reception length does not matter, just
that it is resonant at X mhz - transmit.

VSWR is highly over-rated. The only real reason to keep VSWR low is
that high VSWR will cause the transmitter to protect itself and partly
shut down.

Try this experiment. Take a piece of sheet metal (or aluminum foil
covered cardboard) to act as a ground plane. Insert and SO-239
connector in the middle. Add a length of moderately stiff electrical
wire to the SO-239 that is longer than 1/4 wave at the weather
frequency (162.xxxx). Find a receiver that will measure the actual
receive signal strength. An all mode or AM (not FM) receiver will
work nicely. Extra credit for using a service monitor. Make a
measurement and start cutting the length of the antenna in roughly
1/2" intervals. Measure the receive signal strength.

What I've found when I've done this, is that the antenna gain, which
is what the receive signal level indicates, doesn't change very much
until you get down to about 1/8th wavelength. I modeled this test
using 4NEC2 and found the same thing.

Now, if you believe that the tx and rx performance of an antenna are
identical, this would suggest that you could make the antenna almost
any length, and still have adequate gain and function if you could fix
the VSWR.

Public service here all uses Larsen antenna's - especially the PA State
Police, and they have very deep pockets.

Obviously, the more expensive the antenna, the better it works.

Shakespeare also makes a decent mobile antenna.

They mostly make marine and military antennas. Their commercial
antennas are overpriced versions of the antennas that they sell to the
military. They're very well built, rugged, but not cheap.
http://shakespeare-military.com

You need a antenna analyzer or a Dip Meter to set to resonance.

Have you ever tried to resonate a 1/4 wave antenna with either of
those? You'll find that it's affected by the position and location of
just about everything within about a 20 ft radius. I run a sweep
generator, directional coupler, detector, and scope combination to
test antennas, but no way would I ever use that to tune the antenna.
Just getting near the antenna ruins the display. Incidentally, for
complex antennas, such as a dual band J-pole, minimum VSWR isn't
always at resonance.

The only thing the SWR meter can do is tell you what is happening in the
feed line.

Wrong. A VSWR meter reading is affected by the xmitter output
impedance, feed line impedance to the VSWR meter, characteristic
impedance of the coax cables(s), feed line impedance after the VSWR
meter, and of course, the antenna impedance. That's actually a
problem because a VSWR meter is affected by literally everything.

Drivel: I run mostly 75 ohm systems (because the coax is cheap and
easy and has less loss). I had to build my own 75 ohm directional
coupler in order to get accurate VSWR measurements. (Yes, Bird makes
a 75 ohm wattmeter 4307, but I don't want to spend the money).

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

On 6/26/2013 10:17 AM, Jeff Liebermann wrote:
On Wed, 26 Jun 2013 13:16:39 +0100, Channel Jumper
wrote:

A mobile vertical antenna has no gain - gain is only achieved when you
have gain in one direction and rejection in one or more directions.

Wrong. Gain on a mobile antenna can be achieved without
directionality. Just reduce the vertical radiation angle, which puts
more RF towards the horizon, and less RF towards the sky and the
ground.

The only measurable gain would be gain as compared to a dipole or gain
over isotropic.

Yep. That's the way gain is normally measured. dBd or dBi.

1/4 wave antenna's can sometimes produce a better signal locally,
because the radiation pattern is spread out over a larger area.

I've seen that. However, it's usually the result of misusing a "gain"
type antenna, such as a dual band 5/8 ham antenna being used on marine
or commercial frequencies, or a 5/8 commercial antenna, being used on
ham frequencies. Lots of ways to do it wrong. Where a 1/4 wave
antenna really shines is when one needs to cover a wide range of VHF
frequencies, from aircraft to marine.

It will give no distance gain - just local reception.]

Ummm... the range depends more on the terrain than on the antenna.

Because it is all one radio - the reception length does not matter, just
that it is resonant at X mhz - transmit.

VSWR is highly over-rated. The only real reason to keep VSWR low is
that high VSWR will cause the transmitter to protect itself and partly
shut down.

Try this experiment. Take a piece of sheet metal (or aluminum foil
covered cardboard) to act as a ground plane. Insert and SO-239
connector in the middle. Add a length of moderately stiff electrical
wire to the SO-239 that is longer than 1/4 wave at the weather
frequency (162.xxxx). Find a receiver that will measure the actual
receive signal strength. An all mode or AM (not FM) receiver will
work nicely. Extra credit for using a service monitor. Make a
measurement and start cutting the length of the antenna in roughly
1/2" intervals. Measure the receive signal strength.

What I've found when I've done this, is that the antenna gain, which
is what the receive signal level indicates, doesn't change very much
until you get down to about 1/8th wavelength. I modeled this test
using 4NEC2 and found the same thing.

Now, if you believe that the tx and rx performance of an antenna are
identical, this would suggest that you could make the antenna almost
any length, and still have adequate gain and function if you could fix
the VSWR.



I do not doubt your information here. However, it seems to conflict with
my experiences working 75 meters. I work 75 each day using a 75 meter
horizontal loop. I hear the same characters on each day. Often a newbie
pops up with a poor signal. He is in the same area as "the gang" and yet
his signal stinks. Almost invariably we ask him about his G5RV. "Gee
guys how did you know I was using a G5RV?" Poor signals shows up every
time. He is using a dipole that is way too short to resonate on 75
meters. I think they are 82 feet long. It seems to me if VSWR made
little difference, then his 82 foot long dipole on 75 meters should work
just fine. Not trying for a fight, just want an opinion about why we are
hearing this effect. Of course they are using tuners to make a match to
their transceivers.


My own loop is carefully cut for 3.9 mhz. I need a tuner because it is
feed with 600 ohm open wire line and has a nasty VSWR because of
mismatch between the lead-in and antenna. The online calculator for loss
using my antenna system comes out to be 1/2 db. I can live with that.
However, if I put up a loop that was 1/2 the size I need, and then
matched it with a tuner, it would hardly work at all. I know. I tried
loading mine on 160 meters. I could make a match with the tuner. But it
was a bust.

Public service here all uses Larsen antenna's - especially the PA State
Police, and they have very deep pockets.

Obviously, the more expensive the antenna, the better it works.

Shakespeare also makes a decent mobile antenna.

They mostly make marine and military antennas. Their commercial
antennas are overpriced versions of the antennas that they sell to the
military. They're very well built, rugged, but not cheap.
http://shakespeare-military.com

You need a antenna analyzer or a Dip Meter to set to resonance.

Have you ever tried to resonate a 1/4 wave antenna with either of
those? You'll find that it's affected by the position and location of
just about everything within about a 20 ft radius. I run a sweep
generator, directional coupler, detector, and scope combination to
test antennas, but no way would I ever use that to tune the antenna.
Just getting near the antenna ruins the display. Incidentally, for
complex antennas, such as a dual band J-pole, minimum VSWR isn't
always at resonance.

The only thing the SWR meter can do is tell you what is happening in the
feed line.

Wrong. A VSWR meter reading is affected by the xmitter output
impedance, feed line impedance to the VSWR meter, characteristic
impedance of the coax cables(s), feed line impedance after the VSWR
meter, and of course, the antenna impedance. That's actually a
problem because a VSWR meter is affected by literally everything.

Drivel: I run mostly 75 ohm systems (because the coax is cheap and
easy and has less loss). I had to build my own 75 ohm directional
coupler in order to get accurate VSWR measurements. (Yes, Bird makes
a 75 ohm wattmeter 4307, but I don't want to spend the money).

"seediq" wrote in message
news
I do not doubt your information here. However, it seems to conflict with
my experiences working 75 meters. I work 75 each day using a 75 meter
horizontal loop. I hear the same characters on each day. Often a newbie
pops up with a poor signal. He is in the same area as "the gang" and yet
his signal stinks. Almost invariably we ask him about his G5RV. "Gee guys
how did you know I was using a G5RV?" Poor signals shows up every time. He
is using a dipole that is way too short to resonate on 75 meters. I think
they are 82 feet long. It seems to me if VSWR made little difference, then
his 82 foot long dipole on 75 meters should work just fine. Not trying for
a fight, just want an opinion about why we are hearing this effect. Of
course they are using tuners to make a match to their transceivers.


My own loop is carefully cut for 3.9 mhz. I need a tuner because it is
feed with 600 ohm open wire line and has a nasty VSWR because of mismatch
between the lead-in and antenna. The online calculator for loss using my
antenna system comes out to be 1/2 db. I can live with that. However, if I
put up a loop that was 1/2 the size I need, and then matched it with a
tuner, it would hardly work at all. I know. I tried loading mine on 160
meters. I could make a match with the tuner. But it was a bust.


You can not compair what goes on at 75 meters with 6 meters and above. Less
than 99.9 % of the hams can not put up equal antennas. For the mobile on
VHF it would have to be around 200 feet high and the truck would have ot be
200 to 400 feet wide and long..

I don't like the g5rv either, but they seem to work ok. Your loop works
fine for talking to the same people each day. Try it at other distances.
Going say 3000 miles away, a short vertical may be beter. I don't do much
on 75, but do some on 20 meters. It all depends on the propogation. One
day some stations with beams were hardly workable and a state or two away I
worked a mobile and another with a temporary vertical that was running 5
watts, they were both s9 or beter.

I have played with vhf repeaters for about 40 years. It may depend on the
area you are in as to the best kind of all around vhf antenna. One day a
fellow ham and I rounded up several antennas of all kinds. From 1/4 wave to
one about 6 feet long for 2 meters. There did not seem to be a clear
winner. Even a 40 meter antenna mounted to the bumper that was about 10
feet long worked as well receiving one repeater while the car was parked in
the same spot.
The area around here averages about 700 feet above sea level.

Some of the repeaters are from about the same height to around 5000 feet
above sea level. About the only overall differance we have found is the 5
or 6 foot long antennas do not seem to work very well while in motion and
the 5/8 antennas need to be stiff enough that they do not lay back at
highway speeds.

In areas that are flat it may be a whole differant story and the antennas
that keep the signal near the earth such as a 5/8 may work a lot beter
overall.

We did notice a big differance when going from a 4 bay dipole to a colinear
about the same overall length. The dipole and colinear were both Phelps
Dodge, not the cheap ham antennas. While the rated gain differance was
about 1 db infavor of the dipole aray, the noted coverage was much less when
using the colinear. Years later, we switched back to the dipole aray and
the coverage came back. If used in a differant area, there could be another
differance in coverage when compaired.

On Thu, 27 Jun 2013 13:34:20 -0500, seediq
wrote:

I do not doubt your information here. However, it seems to conflict with
my experiences working 75 meters.

Ummm... is there a weather channel on 75 meters? My "cut the antenna
length" test was intended for VHF, which is a very different beast
from 75 meters. The big difference between VHF and HF is that HF
turns the nearby earth, ground, water table, hills, buildings, fences,
and neighbors into part of the antenna system. For VHF, once I get
out of the near field region, most of that stuff can be ignored
(unless it's also in the Fresnel Zone). I can elaborate more on this
if you like, but I'm not an expert or seriously experience with land
based HF antennas, just marine HF, which is yet a different beast.

I'm stuck at home today with a foot problem. So, I get to sit at the
computer instead of the workbench. I'll throw together a web page
showing that cutting the antenna short does NOT reduce it's gain and
efficiency very much (but does mangle the pattern and VSWR). Stay
tuned.

I work 75 each day using a 75 meter
horizontal loop. I hear the same characters on each day. Often a newbie
pops up with a poor signal. He is in the same area as "the gang" and yet
his signal stinks. Almost invariably we ask him about his G5RV. "Gee
guys how did you know I was using a G5RV?" Poor signals shows up every
time. He is using a dipole that is way too short to resonate on 75
meters. I think they are 82 feet long. It seems to me if VSWR made
little difference, then his 82 foot long dipole on 75 meters should work
just fine. Not trying for a fight, just want an opinion about why we are
hearing this effect.

Can I pass on this right now? I have some definite opinions on HF
antennas and the G5RV, which unfortunately I cannot substantiate with
either experience or calculations. Rather than post erroneous
information, I'll keep my foot in my mouth where it belongs.

However, I can't resist giving you a clue as to what's different
between an excessively short dipole and a real antenna. A hint is
that unless the VSWR is outrageously high or the antenna was made from
barbed wire, nearly 100.0% of the RF that is applied to it gets
radiated in some direction. The some direction is the key. With a
decent antenna, it's going in the right direction. With a not so
wonderful antenna, it's going in useless directions, such as into the
ground. I'll stop there before I get myself into trouble.

Of course they are using tuners to make a match to
their transceivers.

I should also point out that one can always make things worse with an
antenna tuner. Try the loss on 160m and 75m with this Java applet:
http://fermi.la.asu.edu/w9cf/tuner/tuner.html
Tweak the values of Q for the caps and inductors for a more realistic
calculation.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

On Thu, 27 Jun 2013 16:11:50 -0700, Jeff Liebermann
wrote:

I'm stuck at home today with a foot problem. So, I get to sit at the
computer instead of the workbench. I'll throw together a web page
showing that cutting the antenna short does NOT reduce it's gain and
efficiency very much (but does mangle the pattern and VSWR). Stay
tuned.

5 hours (minus dinner) later and I'm dead tired. What started as a
simple little demonstration turned into a time burning nightmare.
Here's where I stopped:
http://www.11junk.com/jeffl/antennas/Monopole/index.html
The various sub-directories are NEC2 models for various length
monopole antennas over a perfect ground plane. That a rough
approximation of what one would expect to see on the roof of a car
with a large metal roof at VHF/UHF frequencies. It's not quite
correct, but close enough for this exercise.

The directories are named after the length of the monopole antenna.
For example:
monopole_0_625
is a 0.625 or 5/8th wavelength antenna. The underscores were used
because Windoze XP detests more than one period in a filename.

The NEC deck is really simple.

CM Monopole antenna over perfect ground.
CM by Jeff Liebermann AE6KS 06/25/2013
CE
SY LENGTH = 0.625 'Length in wavelengths
GW 1 21 0 0 0 0 0 LENGTH 0.001
GE 1
GN 1
EK
EX 0 1 1 0 1 0
FR 0 0 0 0 299.8 0
EN

The only value that changes for each antenna is the label:
LENGTH = X.XXX
The 0.001 is 0.001 wavelengths for a wavelength = 1 meter, which is a
2mm diameter monopole antenna. The 299.8 MHz frequency is a
convenient trick to make 1 wavelength equal to 1 meter, making all the
dimension appear in wavelengths. That allow this antenna to be easily
scaled to any frequency.

If you feel ambitious, download and install 4NEC2 from:
http://www.qsl.net/4nec2/
and try it. If you're really into big models, I suggest you also get
the multi-core/processor NEC2 engine from:
http://users.otenet.gr/~jmsp/
which really speeds things up.

So much for the background stuff...

Start with the 1/4 wave antenna at:
http://www.11junk.com/jeffl/antennas/Monopole/monopole_0_250/index.html
http://www.11junk.com/jeffl/antennas/Monopole/monopole_0_250/slides/monopole_0_250.html
Note that the gain is 5.19dBi. At this point, I usually get an
outrage from everyone who knows that a dipole is 2.15dBi and that this
monopole can't possibly have more. Well, we have a perfectly
reflective ground under this antenna, that reflects 100.0% of
everything that hits it, effectively doubling the gain.
2.15dBi + 3.01dB doubling = 5.16dBi
You'll see the extra 3dB gain throughout the various pages.

The common misconception is that shorter antennas have less gain. Yes,
they do, but it's not really proportional to the length. For example,
the 1/4 wave monopole may have 5.19dBi gain,
http://www.11junk.com/jeffl/antennas/Monopole/monopole_0_250/slides/monopole_0_250.html
but the 1/8th wave monopole still has 4.86dbi gain
http://www.11junk.com/jeffl/antennas/Monopole/monopole_0_125/slides/monopole_0_125.html
and the 1/20th wave monopole still has 4.79dBi gain.

Going the other direction with longer monopole antennas, the full wave
monopole at:
http://www.11junk.com/jeffl/antennas/Monopole/monopole_1_000/slides/monopole_1_000.html
has 7.06dBi gain or less than 2dB more than a 1/4 monopole. One might
expect that having 4 times as much wire as the 1/4 wave monopole would
produce a 6dB gain increase, but that's not how it works.

I did some tweaking and arranged to produce the antenna impedance in
polar form. For example, the 1/4 wave antenna at:
http://www.11junk.com/jeffl/antennas/Monopole/monopole_0_250/slides/monopole_0_250.html
has an impedance of 48.7 ohms with a phase angle of 30.2 degrees.
Close enough to 50 ohms.

However, as we get into even multiples of 1/4 wavelength, the
impedances become very high. For example, the infamous 1/2 wave
monopole shows 934 ohms:
http://www.11junk.com/jeffl/antennas/Monopole/monopole_0_500/slides/monopole_0_500.html
which is not going to be easy to match.

On the short end of the scale, the 1/8th wave antenna at:
http://www.11junk.com/jeffl/antennas/Monopole/monopole_0_125/slides/monopole_0_125.html
shows 254 ohms, which will work with a 2:1 turns ratio transformer.

If you look at the antennas that are odd multiples of 1/4 wavelength,
you'll notice that their impedances are tolerably close to 50 ohms.
For example, the 1.25 wavelength antenna is 72.9 ohms, which will
probably work without any matching xformer.
http://www.11junk.com/jeffl/antennas/Monopole/monopole_1_250/slides/monopole_1_250.html

If you look at the patterns at:
http://www.11junk.com/jeffl/antennas/Monopole/index.html
you'll see some interesting things. The pattern for the 1/2 wave
monopole and shorter are all very similar. The gain is also fairly
constant. I can't say the same for the impedance, which varies
radically and the takeoff angle, which keeps creeping upward as the
antenna gets longer. As the antenna gets really long, such as this 5
wavelength monopole monster:
http://www.11junk.com/jeffl/antennas/Monopole/monopole_5_000/slides/pattern.html
the major lobes are almost straight up, which might be useful for
talking to satellites but not terrestrial repeaters. Note that the
gain has increased to 10.7dBi or 5.5dB more than the 1/4 wave
monopole.

Lots more can be extracted from the simulations. I'll clean up the
mess, contrive a web page, make it pretty, but not tonite.








--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

On 6/27/2013 11:49 PM, Jeff Liebermann wrote:

On the short end of the scale, the 1/8th wave antenna at:
http://www.11junk.com/jeffl/antennas/Monopole/monopole_0_125/slides/monopole_0_125.html
shows 254 ohms, which will work with a 2:1 turns ratio transformer.

I don't think a transformer is a significant help. Without the
transformer the SWR is about 158:1. With the transformer, the SWR is
still up to about 61:1. That will probably kick in the SWR protection of
the transmitter.

John - KD5YI