PMR field measurements - RF Cafe Forums

 Post subject: PMR field measurements

Posted: Thu Mar 05, 2009 8:20 pm 

Hi,

I have a 1/4 wave unity gain roof mounted whip antenna (66-175MHz band) which connects to a unit which takes RF field strength measurements along with GPS readings.

I would like to take measurements in the 80MHz and 160MHz bands and I was wondering if I need to cut the antenna to the appropriate 1/4 wavelength using the relationship

lamda = c/f

where c is the speed of light
f is the operating frequency
lamda is the wavelength of the frequency f

If I just leave the antenna at the default 1.3 metre length what affect will it have on the field strength readings at the different bands?

My test equipment takes measurements in dBm, do I need to include an antenna calibration (K or antenna) factor for the measurements calculated using the equation?

K = L – G + 20log(f) – 32

Where

K is the antenna/K factor (dB)
L is the combined cable and connector loss (dB)
G is the antenna gain (dBd)
F is the frequency (MHz)

Any help is appreciated, I'm a bit new to the RF end of the transceiver chain.

Regards,

Gavin

 Post subject: Re: PMR field measurements

Posted: Fri Mar 13, 2009 3:51 pm 

Hi Gavin
The antenna you mention is too narrow in bandwidth to measure the field strength accurately and use correction formulas across such a wide band: the error in correction factors would be too high.
You either need to cut several vertical monopoles using the formula you quote for wavelength, with a correction factor of say 0.94 to allow for detuning capacitive effects to ground, or buy/construct a broadband antenna. There are some verticals advertised in the amateur press that cover 40 to 1300MHz in one antenna, and they are not expemsive. Professional ones are far more expensive.
Two important factors come into the calculation:
1 the effect of feed line mismatch reflecting some of the signal back into space instead of to the receiver, which needs an accurate impedance plot in order to do a correction. Thus the less variation there is across the band the easier is the correction.
2 gain variation due to changing "aperture" size, which is fairly easy to compensate for.
Take a look around the market for a broadband 50 ohm unbalanced (ie coax feed) antenna that covers the band you want, or if you want to experiment, then built your own.

_________________
At bottom, life is all about
Sucking in and blowing out.

 Post subject: Re: PMR field measurements

Posted: Mon Mar 16, 2009 10:30 pm 

Hi Nubbage,

Thanks for your reply, I think I wasn't clear in my initial post, I am not trying to take measurements across the whole band from 80 to 160 MHz but to take 2 sets of measurements, one in each band. The channel width of both bands is 12.5kHz.

With this in my mind, do I need to address any of the issues raised in my initial post?

Regards,

Gavin

 Post subject: Re: PMR field measurements

Posted: Tue Mar 17, 2009 10:10 am 

Hi Gavin
No, I understood your point. My suggestion to find a single antenna that covers the 2 bands of interest is then
a) you do not have to change antennas as you do the drive test and
b) the supplier will give you an antenna calibration factor for the antenna
A low-cost solution would be to cut quarter-wave verticals for each of the bands using that formula for wavelength, adjusted short by 5%. Confirming the true gain is then not so straight-forward, however.
I have never used/needed the formula you quote. When calibrating I use a measurement point near to (a known distance) within line of site of my base station, and compute what the signal level should be from the transmit output power, feeder loss, and Tx antenna gain, Rx antenna gain and any feeder loss. I have also calibrated the receiver from a recently calibrated signal generator, giving an accurately known output power in dBm. The computed and measured signal levels will be slightly different due to second order effects like feeder impedance mis-match loss etc. Typically this difference is 1 or 2dB, which for a bog standard drive test is usually good enough. Then away I go letting the 2 channel logger (GPS and signal level) chug away.

_________________
At bottom, life is all about
Sucking in and blowing out.

 Post subject: Re: PMR field measurements

Posted: Mon Mar 23, 2009 2:51 am 

Thanks for the clarification, will be taking measurements this weekend.