Antennas for Field Measurement

AM Radio discussion. Directional arrays are FUN!
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frank2017
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Antennas for Field Measurement

Post by frank2017 » Wed Jul 12, 2017 9:35 pm

If I want to carry field measurement with the spectrum analyzer, what types of antennas am I looking for? A calibrated shield loop? Active or Passive?
How about for drive test measurement around my town, a monopole? How's the sensitivity of monopole on AM? Do I need an active one?

Appreciate for your help. :wink:

ChuckG
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Re: Antennas for Field Measurement

Post by ChuckG » Wed Jul 12, 2017 10:08 pm

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Chuck Gennaro
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davedybas
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Re: Antennas for Field Measurement

Post by davedybas » Sun Jul 16, 2017 12:35 pm

Frank,

AM signal Measurements are done with a Field Strength Meter designed for that type of measurement. Potomac Instruments makes that type of meter, and it is considered the standard test equipment for that use.

AM signal measurements are rarely made from a car with an antenna stuck on it.... There are too many factors that could affect the reading and one cars rooftop measurement may vary greatly from another persons car's rooftop. Driving under power lines affects an AM stations signal strength at that location, driving past tall metal poles affects the readings at those locations.

Typically AM signal measurements are made with a proper calibrated field strength at locations that are free from any power lines or metal structures. The results of the measurements are plotted on a graph.

But nothing is to stop you from putting an antenna on your car and using a signal spectrum analyzer. However the results can't really be used as a reference.

Typically a Spectrum Analyzer & Shielded Loop antenna are used for NRSC signal bandwidth measurements at a single location.

frank2017
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Re: Antennas for Field Measurement

Post by frank2017 » Mon Jul 17, 2017 9:03 am

Thanks for the advice and detailed explanation. I'm very new to AM. :wink:

I thought with the spectrum analyzer and a calibrated antenna, I can have an accurate field strength reading just like the Potomac FIM.
Besides, are there any guidelines or standards for measuring AM field strength? I want to verify the performance of my antenna.

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kkiddkkidd
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Re: Antennas for Field Measurement

Post by kkiddkkidd » Mon Jul 17, 2017 9:36 am

You can get a relative measurement but not an absolute measurement without a truly calibrated receiver and matching calibrated antenna. Hence the Field Intensity Meter or FIM. You could certainly take measurements with a specan and loop or whip and compare your signal against another similar signal at the same power, distance and environment (which I have done). However, those measurements would be hard to quantify into something meaningful that could be compared to an old proof or charts. Or at least I know of no way to do so. If there is, I would be interested in hearing about it myself...

FIM's have an internal calibration oscillator and use a carefully calibrated, internal, purpose built antenna to take meaningful absolute measurements. Even at that, measurements can be quite subjective due to environment, terrain, reradiation, shielding, day of week, moon phase, etc. OK, probably not day of week or moon phase but you get the idea.

Regards,
frank2017 wrote:
Mon Jul 17, 2017 9:03 am
Thanks for the advice and detailed explanation. I'm very new to AM. :wink:

I thought with the spectrum analyzer and a calibrated antenna, I can have an accurate field strength reading just like the Potomac FIM.
Besides, are there any guidelines or standards for measuring AM field strength? I want to verify the performance of my antenna.
--
Kevin C. Kidd CSRE/AMD
WD4RAT
AM Ground Systems Company
http://www.amgroundsystems.com
KK Broadcast Engineering
http://www.kkbc.com

frank2017
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Re: Antennas for Field Measurement

Post by frank2017 » Tue Jul 18, 2017 1:34 am

kkiddkkidd wrote:
Mon Jul 17, 2017 9:36 am
You can get a relative measurement but not an absolute measurement without a truly calibrated receiver and matching calibrated antenna. Hence the Field Intensity Meter or FIM. You could certainly take measurements with a specan and loop or whip and compare your signal against another similar signal at the same power, distance and environment (which I have done). However, those measurements would be hard to quantify into something meaningful that could be compared to an old proof or charts. Or at least I know of no way to do so. If there is, I would be interested in hearing about it myself...

FIM's have an internal calibration oscillator and use a carefully calibrated, internal, purpose built antenna to take meaningful absolute measurements. Even at that, measurements can be quite subjective due to environment, terrain, reradiation, shielding, day of week, moon phase, etc. OK, probably not day of week or moon phase but you get the idea.

Regards,
frank2017 wrote:
Mon Jul 17, 2017 9:03 am
Thanks for the advice and detailed explanation. I'm very new to AM. :wink:

I thought with the spectrum analyzer and a calibrated antenna, I can have an accurate field strength reading just like the Potomac FIM.
Besides, are there any guidelines or standards for measuring AM field strength? I want to verify the performance of my antenna.
Thanks for the reply.
I never tried AM field measurement :oops: , what do you mean old proof and charts? Is it the predicted antenna pattern? :roll:
What antennas you used for the relative measurement? any brand and model?

kcbooboo
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Re: Antennas for Field Measurement

Post by kcbooboo » Tue Jul 18, 2017 5:44 am

I believe Kevin was referring to the previous or original proof-of-performance measurements that had to be done when the station went on the air. Those are your reference, and the signal levels were surely made with an FIM.

I have an FIM and a spectrum analyze and Scott loop antenna. There is a calibration chart that comes with the antenna. You get a multiplication factor that you multiply the signal level by, to get the true level at a specific frequency. Even with that, the readings are never the same. For one thing, the carrier level moves up and down with modulation, depending on the program and how the station has set up their positive modulation. The SA will pick this up and that peak will keep moving. You can set the SA for peak hold, averaging, or something else. I don't know which one matches the FIM's meter response, but I never see the FIM's needle jumping around as much as the SA's carrier peak does.

The FIM gives you a readout directly in microvolts, millivolts, or volts per meter. The SA gives you a reading usually in dBm but some analyzers let you convert that to volts. With the calibration factor of the antenna, you can get the same microvolts, millivolts, or volts per meter. One 5kw station I do spectrums for produces about 600mV/m at 1kw, and that ends up being around -43dBm on the SA using the loop antenna. You'll find that the weaker signals may be down in the noise when using the SA and loop antenna, and will be much harder to see. The FIM can detect signals down to 10 microvolts per meter, which is VERY weak. The SA / loop setup is really best when you're in a relatively strong location, such as 100mV/m or greater.

You certainly can come up with your own calibration factor for any antenna if you have some reference to compare it to, and the best tool for that is the good old FIM. By the way, Nems-Clarke made their 120-series of FIMs that only cover 540-1600 kHz. RCA put their name on some, the WX-2 series. These would be fine for your task. Surprisingly they're quite accurate considering their age (made in the 1950s). Unfortunately no one calibrates them any more but they do show up used from time to time, mainly because they can't check harmonics above 1600 kHz.

Bob M.

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kkiddkkidd
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Re: Antennas for Field Measurement

Post by kkiddkkidd » Tue Jul 18, 2017 12:28 pm

Frank,

The proof that I referred to is the "Proof of Performance" that is required for all directional arrays and some non-directional sites. As Bob said, those measurements would have been made with an accepted FIM. The Charts that I referred to are the FCC field strength/ground conductivity charts. You would have to have a calibrated FIM to get anything meaningful from those as well.

As far as relative measurements go, any antenna would be fine for that. Before I got one of the Chris Scott loop antenna's for NRSC testing, I used a simple vertical stainless whip about 5ft long with no tuning whatsoever. It worked fine but was fairly deaf (very, very short for the frequency) and very susceptible to locally generated noise from everything.

Assuming that your station is non-Directional (ND), if there is another similar station in your area on a reasonably close frequency, one of the simplest ways to compare your non-directional signal is to go to a clear spot that is equidistant from both transmitters and simply compare the signals from your site and the other one on the spectrum analyzer. You will need to do this in several equidistant locations to help average out any environmental errors. This assuming that both stations have similar operating parameters (freq, power, tower height, ND, etc).

This isn't going to give you a huge amount of info at the best and will tell you absolutely nothing meaningful if the stations aren't very, very similar or are either is directional.


frank2017 wrote:
Tue Jul 18, 2017 1:34 am

Thanks for the reply.
I never tried AM field measurement :oops: , what do you mean old proof and charts? Is it the predicted antenna pattern? :roll:
What antennas you used for the relative measurement? any brand and model?
--
Kevin C. Kidd CSRE/AMD
WD4RAT
AM Ground Systems Company
http://www.amgroundsystems.com
KK Broadcast Engineering
http://www.kkbc.com

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