Hello and greetings all,

I borrowed a field strength measurement equipment from the school lab.
I wanted to measure the the field strength of a low power FM
transmitter. I was told that I need to setup the antenna height
correctly. I didn't think of this. I thought all I need is the proper
antenna length and polarization for measurement. Can anyone give me
some insight regarding using the proper antenna height? Thank you.

Ron J wrote:

Hello and greetings all,

I borrowed a field strength measurement equipment from the school lab.
I wanted to measure the the field strength of a low power FM
transmitter. I was told that I need to setup the antenna height
correctly. I didn't think of this. I thought all I need is the proper
antenna length and polarization for measurement. Can anyone give me
some insight regarding using the proper antenna height? Thank you.


Ron,

You need to tell what antenna type it is first.
Some antennas need to be 1/4 of a wavelength up in the air to work properly,
other types maybe 1/2 wavelength up.
Is it a vertikal, dipole or what...

Many antennas develops a high angle of rtadiation when close to the ground
and after you mounted the antenna at the correct height, a totally
different radiation pattern develops.

So there are no simple answer to your question Ron, not without more
information.

Cheers

Dan / M0DFI

Dan Andersson wrote:
Ron J wrote:

Hello and greetings all,

I borrowed a field strength measurement equipment from the school lab.
I wanted to measure the the field strength of a low power FM
transmitter. I was told that I need to setup the antenna height
correctly. I didn't think of this. I thought all I need is the proper
antenna length and polarization for measurement. Can anyone give me
some insight regarding using the proper antenna height? Thank you.

So there are no simple answer to your question Ron, not without more
information.
Cheers
Dan / M0DFI

Generally when you place TX and RX antennas above the ground
and you change the height of RX antena, you see the variation of
signal strength. This is due to the ground reflection and we call
it "height pattern." Probably your manual suggests this influence.
How to avoid this? (although both methods do not give rigorous results,)
1. You may take the average of height patten.
2. Make the distance short between the antennas.

Tom/jj3fbs

Dan, It is a dipole antenna.

Thanks!

Tom, can you clarify on how to take the average height of the pattern?
Also, if I keep the TX and RX antenna close, wouldn't that eliminate
the far-field effects?

Thanks!

Tom, can you clarify on how to take the average height of the pattern?

It is not the average height. It is the average reception power.
Suppose you change the height of RX antenna (preferably together with
TX antenna) by (more than) a half wave length, you'll observe maximum
level Px and minimum level Pn.
So the reception voltages Vx and Vm are sqrt(Px) and sqrt(Pm).
The average voltage Va is then (Vx+Vm)/2. Thus average RX power is Va^2.

Also, if I keep the TX and RX antenna close, wouldn't that eliminate
the far-field effects?

It is not a far-field measurement. But the error might not be as
much as 3dB if the distance is such as two to three wave-lengths,
which is sometimes better than the bad average measurement performance
or doing nothing.

Tom/jj3fbs