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

On Sun, 30 Jun 2013 07:24:34 -0500, John S
wrote:

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

Nope. A 2:1 turns ratio tranformer will provide a 4:1 impedance
ratio, not a 2:1 impedance ratio.

The required transformer ratio would be:
(254 / 50)^0.5 = sqrt(5) = 2.3
A 2:1 turns ratio xformer should be close enough.

Another way is to take the 2:1 turns ratio transformer, which has a
4:1 impedance ratio, and divide the antenna impedance by the impedance
ratio:
254 / 4 = 63.5 ohms.
Not exactly 50 ohms, but close enough.






--
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 Sun, 30 Jun 2013 10:58:08 -0700, Jeff Liebermann
wrote:

On Sun, 30 Jun 2013 07:24:34 -0500, John S
wrote:

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

Nope. A 2:1 turns ratio tranformer will provide a 4:1 impedance
ratio, not a 2:1 impedance ratio.

The required transformer ratio would be:
(254 / 50)^0.5 = sqrt(5) = 2.3
A 2:1 turns ratio xformer should be close enough.

Another way is to take the 2:1 turns ratio transformer, which has a
4:1 impedance ratio, and divide the antenna impedance by the impedance
ratio:
254 / 4 = 63.5 ohms.
Not exactly 50 ohms, but close enough.

Oops. My mistake. I couldn't recall if a 2:1 transformer referred to
the turns ratio or the impedance ratio. I've seen it done both ways
in other industries and transformer applications. I usually qualify
the label with either turns or impedance ratio but forgot this time.
However, skimming the available literature with Google, I find that
the common usage for RF xformers is the impedance ratio. Therefore,
your comments are correct and I should have specified a 4:1
transformer. Sorry(tm).



--
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 6/30/2013 1:05 PM, Jeff Liebermann wrote:
On Sun, 30 Jun 2013 10:58:08 -0700, Jeff Liebermann
wrote:

On Sun, 30 Jun 2013 07:24:34 -0500, John S
wrote:

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

Nope. A 2:1 turns ratio tranformer will provide a 4:1 impedance
ratio, not a 2:1 impedance ratio.

The required transformer ratio would be:
(254 / 50)^0.5 = sqrt(5) = 2.3
A 2:1 turns ratio xformer should be close enough.

Another way is to take the 2:1 turns ratio transformer, which has a
4:1 impedance ratio, and divide the antenna impedance by the impedance
ratio:
254 / 4 = 63.5 ohms.
Not exactly 50 ohms, but close enough.

Oops. My mistake. I couldn't recall if a 2:1 transformer referred to
the turns ratio or the impedance ratio. I've seen it done both ways
in other industries and transformer applications. I usually qualify
the label with either turns or impedance ratio but forgot this time.
However, skimming the available literature with Google, I find that
the common usage for RF xformers is the impedance ratio. Therefore,
your comments are correct and I should have specified a 4:1
transformer. Sorry(tm).

No problem and no reason to apologize.

For the sake of those who read this forum, I will provide my analysis
upon request.

John - KD5YI

On Thursday, June 27, 2013 1:34:20 PM UTC-5, seediq wrote:


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.

A high SWR on it's own is not always bad news. It depends
on the feed line used, freq, etc.
The main reason the G5RV's don't do so hot is the goofy
method of feeding most seem to use. IE: a length of twin lead
to a choke, to coax. And then some add insult to injury and
run a tuner at the shack. A good amount of power is turned to
heat. But if you feed the same antenna with ladder line the
whole route, the losses are not so bad, even with a high SWR.
If you tune the line and use no tuner, fairly low losses.
If you use a tuner, not quite as good, but not too bad if you
use the least amount of inductance needed to get a usable match.
All antennas will radiate nearly all power applied to them.
The trick is getting it from the rig to the antenna without
turning some into heat. This is where the usual G5RV is failing.
Some of the power is not making it to the antenna due to obtuse
lossy methods of feeding. Same issue with some of the windoms, etc
that are sold.

wrote in message
...
A high SWR on it's own is not always bad news. It depends
on the feed line used, freq, etc.
The main reason the G5RV's don't do so hot is the goofy
method of feeding most seem to use. IE: a length of twin lead
to a choke, to coax. And then some add insult to injury and
run a tuner at the shack. A good amount of power is turned to
heat. But if you feed the same antenna with ladder line the
whole route, the losses are not so bad, even with a high SWR.
If you tune the line and use no tuner, fairly low losses.
If you use a tuner, not quite as good, but not too bad if you
use the least amount of inductance needed to get a usable match.
All antennas will radiate nearly all power applied to them.
The trick is getting it from the rig to the antenna without
turning some into heat. This is where the usual G5RV is failing.
Some of the power is not making it to the antenna due to obtuse
lossy methods of feeding. Same issue with some of the windoms, etc
that are sold.


I have not used the g5rv except for some the club uses at field day. From
what I understand about them, they were designed to work on 20 meters. It
was just luck that they will have a reasonable low swr on other ham bands.
If the swr goes up over 3 or 4 to 1 I can see lots of power being lost in
the coax part.

I use a home made version of the off center fed. I can compair it to an 80
meter dipole and a triband beam. It usually matches the dipole or is
sometimes beter depending on the direction of the other stations on 80
meters. On 20 and 10 meters the beam is usually much beter, but if a
station hapens to be in certain places there is not too much differance .
The ocf does not work very well on 15, but it is not suspose to.
All antennas are fed with Davis Bury flex rg 8 type which does not have too
much loss.