On Nov 9, 1:23 pm, Tom Horne wrote:
Is it possible to ask questions here without triggering an arcane debate
about competing views of theory. I'm about to find out. I asked
earlier in another thread what measuring instruments I would need to
have the use of in order to compare the effective radiated power of
different antennas. As near as I can tell there was no answer.

I built a collinear J pole using copper tubing. I'd like to know if it
is more or less effective at radiating whatever works to the stations
I'd like to be able to talk to under conditions of emergency operation
then say a collinear ground plane or any other omni directional antenna.

I would like to deploy the most effective practical antennas that field
testing can devise and not have to wait until the next breakthrough in
physics to be able to get my local governments Email out to my county's
government, the state government and the responding relief forces.

My question, again, is what measuring instruments can be effectively
applied to the comparison to provide results that will be born out by
real world performance. I have to admit that I find the endless
theoretical debate wearying. As long as it continuous then the
newsgroup will be useless to newer licensees, like my self, who would
like to get some "patient council to the beginner" from those of you who
have been there and done that. Before I have to go there and do that
would be soon enough.
--
Tom Horne, W3TDH

Tom,
I feel that the best results are achieved when measuring the field
strength/receiving effectiveness at a distance far enough away to
avoid near-field effects.
I have used a spectrum analyzer connected directly to the antenna to
make the measurements. Most units can read out directly in dB.
First, I had a friend that was about 2 miles away transmit and I
measured the amplitude of his signal at my home using my different
antennas. I was comparing a 1/4 wave ground plane to a homebrew 5
element beam.
We then reversed the setup, where he took the analyzer to his house
and measured the amplitudes of me transmitting using the different
antennas (same power output, of course)
Antenna gain difference and front/back ratio if it is a beam, are
easily measured. In my case, measurements were very close to
theoretical.
This is an expensive piece of test equipment, but someone in the area
may have access to one.

On the other hand, if you are only looking for seat-of-pants
measurements, find a few hams in the area with analog s-meters and
have them give you relative signal strength readings for your
different antennas. No cost, and the real test is whether you can
communicate effectively. We often send out a rover in a car and do
signal strength comparisons throughout the valley so we know where our
signal needs improvement.
Good luck.
Gary WA7MLK

Please guys
Without going to war with each other over the answer and leaving me not
knowing who to believe, is an MFJ analyzer a good choice in the under
five hundred dollar range? Would using one of the one watt HTs do for a
signal source or is that still to high.
--
Tom Horne

Tom,

A one watt HT will do fine, but the signal will still be too strong close
in to work with. You need to get the power down to perhaps one milliwatt
or less to plot the antenna pattern in a field or car park. You can make
up an attenuator to reduce the power from the HT. Just making up a patch
lead between the HT and the antenna with a 50 ohm, 1 watt resistor
shorting the core and outer will probably reduce the signal to something
you can work with while still giving the transmitter a load to work into.
(You can make up exactly 50 ohms using two 100 ohm, 1/2 watt resistors).
Or make up a simple single transistor 'bug' transmitter from a handful of
components. Plenty of designs available through Google No need to spend
more than a couple of dollars. The 9v battery is likely to be the most
expensive bit.

Mike G0ULI

Mike's right - but if you don't have an MFJ 259 antenna analyzer yet, this
would be a good excuse to go ahead and get one. Its just great when working
with antennas.
You can get one for about half of your $500 budget.

Tom Horne wrote:
I asked
earlier in another thread what measuring instruments I would need to
have the use of in order to compare the effective radiated power of
different antennas. As near as I can tell there was no answer.

A Palomar PFS-1 will do what you need done.
Unfortunately, they are out of production.
--
73, Cecil http://www.w5dxp.com

"Hal Rosser" wrote in message
...


Please guys
Without going to war with each other over the answer and leaving me not
knowing who to believe, is an MFJ analyzer a good choice in the under
five hundred dollar range? Would using one of the one watt HTs do for a
signal source or is that still to high.
--
Tom Horne

Tom,

A one watt HT will do fine, but the signal will still be too strong close
in to work with. You need to get the power down to perhaps one milliwatt
or less to plot the antenna pattern in a field or car park. You can make
up an attenuator to reduce the power from the HT. Just making up a patch
lead between the HT and the antenna with a 50 ohm, 1 watt resistor
shorting the core and outer will probably reduce the signal to something
you can work with while still giving the transmitter a load to work into.
(You can make up exactly 50 ohms using two 100 ohm, 1/2 watt resistors).
Or make up a simple single transistor 'bug' transmitter from a handful of
components. Plenty of designs available through Google No need to spend
more than a couple of dollars. The 9v battery is likely to be the most
expensive bit.

Mike G0ULI

Mike's right - but if you don't have an MFJ 259 antenna analyzer yet, this
would be a good excuse to go ahead and get one. Its just great when
working with antennas.
You can get one for about half of your $500 budget.


Tom

It has just occurred to me that if you can make or get hold of a switched
attenuator to stick in between the antenna and the input socket of your
remote receiver, you can make very accurate measurements indeed. I am
thinking of the type with 8 or 10 switches. The first switch gives 1dB of
attenuation, the next 2dB, 4dB, 8dB, and so on. So long as you have some
sort of signal strength meter you can monitor on the receiver, you just
switch in enough attenuation to give the same meter reading at each test
location and record how much attenuation you have switched in at that point.
The more attenuation, the better the received signal. That will allow you to
determine relative signal strength to within 1dB which is going to be good
enough for your purposes. The attenuator can be used for all kinds of
projects, so it might be worth taking the time to build one irrespective of
what you end up using for a signal source. The usual Google search will turn
up construction details, just resistors and switches in a screened box with
some PCB offcuts or copper foil to provide internal screening between each
section.

I agree with Hal, the MFJ kit is jolly good for the price. It does what it
says on the box, just don't expect miracles.

Mike G0ULI

I agree with Hal, the MFJ kit is jolly good for the price. It does what
it says on the box, just don't expect miracles.
===================
Endorse that. Using the MFJ259B I have learned a lot about antennas and
matching units (ATUs) ,not just antenna gain ,but also antenna bandwidth
and (for HF freqs) dial settings for matching units.
Whereas the quality and uncertainty figures of the analyser might be
frowned upon by 'professionals', it an excellent device for any radio
amateur climbing the knowledge ladder.

Frank GM0CSZ / KN6WH

On 10 Nov, 04:17, Highland Ham
wrote:
I agree with Hal, the MFJ kit is jolly good for the price. It does what
it says on the box, just don't expect miracles.

===================
Endorse that. Using the MFJ259B I have learned a lot about antennas and
matching units (ATUs) ,not just antenna gain ,but also antenna bandwidth
and (for HF freqs) dial settings for matching units.
Whereas the quality and uncertainty figures of the analyser might be
frowned upon by 'professionals', it an excellent device for any radio
amateur climbing the knowledge ladder.

Frank GM0CSZ / KN6WH

Why not compare in the real world
Only the poster knows the conditions that he is likely to operate in
and it appears that it is in a very mixed environment.
In such a case I would compare antennas at home or some place
and move the frequency generator around to desired situations.
The generater can be a hand held or anything for that matter.
Now the real world does not care for "s" meters so one would
switch off the limitor in the radio and use a db counter at
the speaker.These results can be graphically recorded for direct
antenna comparison and for the record.
A sound DB counter can be obtained very cheaply
on E bay and the mechanics of comparison are in situations that
only the poster can determine. Lets face it , communication
is measured from what comes out of the speaker.
It is not rocket science!
Art

Now spouting way too much theory to support my suggestions...

Antenna effectiveness is always affected grossly by physical location,
height above ground and nearby obstructions. If you don't know where you
need to setup in advance I assume you want to take some measurements and
thus coverage predict when you do.

I'll stick to VHF/UHF systems only..

Antenna systems are relatively easy to predict performance of. Assuming
you follow known gain figures, the only big problem (IMO) you will
strike is excessive skying of radiation due to feedline etc radiation.
This is commonly cured by "decoupling the line" through some means. -
but I digress.

(Its strange you are looking at emergency comms, that is what the pager
service was I worked on!)

I disagree that max ERP is undesirable. I assume of course you are
talking omnidirectional antennas that tend to compress the vertical
beamwidth. About the only time this isn't desirable is if you are in
high mountainous country and need to either radiate into valleys or gain
reflections from high angles.

I can see the problem you are trying to resolve now. I think however you
need to take a multi tier approach and not just rely on a one time
measurement at a test site. There are just too many variables to allow
for when you move to a "real" location.

Do a number of basic field strength tests in variable topography at
"normal" operating distances and maybe 3-4 locations with a calibrated
measuring system. It doesn't have to be calibrated to an absolute figure
but you need to be able to replicate the process from on test to the
next. At some stage you will be able to create a table of -dBm vs
whatever device you are using for measurement.

If you want to be pedantic play around with likely base antenna mounting
height and method.

Make sure you do a distribution or at least minimally an average
measurement over several wavelengths. When you come back and do the
other antennas use the same measuring location. (A distribution will
also give you an idea how "choppy" the signal will become for a mobile
station)

Weather conditions may also influence results so try and do them at
close to the same time/day

By variable topography I am talking a hill top, flat area and then a valley.

You'll now have some operating distance parameters that you can plug
into a RF coverage program (like RadioMobile). You should be able to
work backwards from the figures you got in the field to establish the
actual antenna gain and radiation angle/lobe etc characteristics. You
will even see the slight bump in the horiz plane pattern of a jpole.

The next step now in setting up for real world is to take the known
antenna parameters and model actual locations that you need to cover for
the emergency. IMO this will give you a much better idea of what your
coverage will be without needing to do actual site measurements.

In other words you have now characterized your antennas and used a PC to
establish what the coverage will be.

When you want to compare another antenna you'll need to go back to your
test site for the greatest accuracy.

I assume you have done coverage modeling. The link below is not a good
representation but will give you an idea of what the output looks like.
In this case it is a 25W base to mobile 2m setup with a 5/8 on the car
and 6dB collinear at the base. The base is off to the upper right of the
image, the map is about 25 miles square and dBm is the scale on the top
left. It is Tyler TX.

http://pages.suddenlink.net/vk2yqa/f...in%20Tyler.jpg

I hope you find this useful. I believe it far more accurate and useful
for your application than comparing antenna ERP by itself.

Cheer Bob W5/VK2YQA

Tom Horne wrote:
As you can see from some of the replies I gave to others I'm trying to
devise a way of practically comparing antennas available because in
emergency service communications support we have no way of knowing were
we will need to set up. Hence the desire to set up some sort of antenna
experiment that will allow us to compare the antennas against each other.

Just for the sake of my education is it likely to be true that the
antenna that puts out the most effective radiated power will be a bad
choice in a large percentage of possible sites?
--
Tom Horne, W3TDH

On Sat, 10 Nov 2007 04:13:56 GMT, Tom Horne
wrote:

I'm trying to
devise a way of practically comparing antennas available because in
emergency service communications support we have no way of knowing were
we will need to set up.

Hi Tom,

There are one, two, three or four certainties about antenna
performance. There are dozens of factors that are way beyond your
control that degrade system performance. Emergencies rarely conform
to optimization; instead you need to think of flexibility.

Hence the desire to set up some sort of antenna
experiment that will allow us to compare the antennas against each other.

As has been offered by a multitude here, that is both very simple in
description and complex in accomplishment.

Just for the sake of my education is it likely to be true that the
antenna that puts out the most effective radiated power will be a bad
choice in a large percentage of possible sites?

This question alone reveals a most curious idea. First, it presumes a
fact that has never, or should have never grown in your mind from all
the contributions here (or from external study):
Effective = Bad
is a non-starter.

Communications performance is measured by link budgets, not antennas
alone. The link budget is an accumulation of factors such as:
1. Applied Power;
2, Transmission line loss;
3. Antenna Gain (Effective Radiated Power);
4. Path Loss;
5. Receiver Sensitivity;
6. Multipath Sensitivity;
7. Noise in receive path.

This list could be made longer, but as long as it is, in an emergency
you really have no control over 4, 5, 6, and 7 (and you may be at some
risk even with 2 and 3). Your task as an emergency operator would be
to recognize and compensate for them as best as possible where it does
not jeopardize mission. Often, mission will negate any opportunity to
do anything about these last factors. This requires you to plan ahead
so that you recognize where these factors could occur and avoid them
first, rather than being distracted with them after their discovery.

The difference between a J-Pole's performance measured on a range, and
that of the standard ground plane is really negligible in comparison
to putting either antenna into a Fresnel Zone where the multipath
completely nulls the signal. They are BOTH dummy loads in that
situation. So you carry a yagi to compensate and switch out the
J-Pole or ground plane.

Unfortunately, you may not know where your contact is and you point
the "optimal" antenna in the wrong direction. The best antenna does
not supply the best result - but that is not a function of the
antenna, but rather the operator (pilot error).

In a nutshell, the questions you are asking imply you are seeking
assurance for managing risk, risk that is so variable that no
assurance is possible. Links fail in the face of best efforts, that
is why it is a hobby at our level of cash flow. $500 is not much of a
premium payment for some emergencies.

The emergency repeater systems I've worked on have represented many
10s of thousands of (1970s) dollars as built up from surplused (MASTR
II and similar) equipment. We spent more like thousands of (2007)
dollars to get there.

73's
Richard Clark, KB7QHC

"Tom Horne" wrote in message
news:8eaZi.83$Y32.5@trnddc04...
Tom Horne wrote:
My question, again, is what measuring instruments can be effectively
applied to the comparison to provide results that will be born out by
real world performance.

Bob Bob wrote:
Hi Tom

Direct field strength measurement at the "normal" coverage distances,
calibrated and compared against a known/real world system is IMO the
best choice. I was involved in a VHF paging project that used a laptop,
GPS and measuring receiver for the job. The laptop had a A/D converter
attached to the parallel port. This gave coverage results that were
compared against a modeled prediction, but there is no reason you
couldn't set it up to compare a "new" system to an existing/real one.
One of the beauties of sampling over some time/distance is that small
positional errors with nulls/peaks evident on VHF/UHF can be averaged or
even studied as a distribution. The system I worked with you could even
see Raleigh fading on, but for us it wasn't a useful output!

Biggest hurdle is the RX. You need some kind of Volts per dBm signal
output. You could of course take an S meter output and calibrate it.

If you want a rough answer it may even be worthwhile attaching a laptop
line input to an RX audio out and doing a visual/waterfall analysis of
the level of (FM) quieting present with different antenna systems. You
could of course also calibrate this system.

If you don't want to travel to the limits of the coverage area you can
always do the tests at a lesser distance and then extrapolate with some
RF coverage software.

Hope you find this helpful. Your comments on theoretical debates are
noted, but the best you can do is to just not read them.

Bob VK2YQA

Bob
As you can see from some of the replies I gave to others I'm trying to
devise a way of practically comparing antennas available because in
emergency service communications support we have no way of knowing were we
will need to set up. Hence the desire to set up some sort of antenna
experiment that will allow us to compare the antennas against each other.

Just for the sake of my education is it likely to be true that the antenna
that puts out the most effective radiated power will be a bad choice in a
large percentage of possible sites?
--
Tom Horne, W3TDH

Hi Tom

My approach to the problem of comparing antennas to each other would
involve using satellite signals as the illuminator and build as many test
antennas as you have interest in.

For 2 meter antennas, the 137 MHz from the NOAA satellites is probably
close enough. That would require making some test antennas about 5% bigger
than the 2meter antennas.

If you E-mail me I can show you some radiation patterns I have plotted
from NOAA satellites. My plots of actual measured signal strength make me
more and more confident that EZNEC is accurate.

Jerry

"Jerry Martes" wrote in message
news:ydoZi.102$RR1.77@trnddc02...

"Tom Horne" wrote in message
For 2 meter antennas, the 137 MHz from the NOAA satellites is probably
close enough. That would require making some test antennas about 5%
bigger than the 2meter antennas.

If you E-mail me I can show you some radiation patterns I have plotted
from NOAA satellites. My plots of actual measured signal strength make
me more and more confident that EZNEC is accurate.

Jerry

With all the OSCAR satellites up there is no need to do go to the NOAA in
the 137 mhz range. The two meter sats will do just fine.

Just because an antenna works well on a sat is no reason to assume it will
work well on signals from the ground. I have not used one , but the old
Ringo antenna sent most of its signal up at an angle. It would probably
make a good sat antenna, but a poor antenna for ground work.

People in this thread are making way too much out of it. In most cases the
longer/bigger the antenna is , the more gain it will have. Just put up the
biggest one of good quality you can and don't worry about it. There will
be enough differance in the lay of the land to make differant antennnas work
beter in differant directions unless you are on a very flat land.