Determining gain of an field installed antenna

[UtilitEE]

I am trying to determine the system gain (gain from the output of the transceiver to the output of the antenna) of an existing antenna system used for both transmitting and receiving. The antenna has a tunable coupler between the transceiver and antenna. What is the best procedure? I also need to verify the gain for both directions (TX/RX)

 

[UtilitEE]

More info on the system, the antenna is an omnidirectional 35 foot monopole.

 

[Higgler]

Normal results when talking about gain comparisons need to set up a calibration unit with a known transmit power with known antenna gain. After initially using that that, then change to your system and compare measured power levels.

Your phrase system gain needs more details. What is your systems? a communications device, like Ham radio at low frequencies and you are trying to evaluate how good it is?

35 foot monopole is 75 foot wavelength, are you working at 13 MHz?

Kevin

 

[VEBill]

Normally you'd simply look up the specifications, and then correct for any marketing hype.

For system-level analysis you had better take into account the actual pattern (elevation pattern in your case). There's no point having +x.x dBi peak gain if this peak gain isn't aimed in the correct direction for your application.

We will need slightly more information about your monopole.

If it is an HF system, then perhaps the antenna is a 1/4 wave (about 6-7 MHz) monopole. If so, then the ground plane (or lack) will have a significant effect.

Also, if the frequency is below about 6 MHz, then the antenna is 'short'. It's efficiency will drop off at lower frequencies. System gain will vary with frequency. Gain in the direction you require will vary wildly with frequency as the antenna becomes long. At higher frequencies the peak gain direction might be aimed quite high.

If the system is V/UHF, then perhaps your "monopole" (sic) is a stacked colinear array - high gain but omnidirectional. That's why we'll need additional info.

Tunable couplers (sounds HF to me...) efficiency will vary with load, and load varies with frequency.

 

[UtilitEE]

The frequencies are from 2 to 3 MHZ (more MF than HF, so it is 'short'). I can measure the power out of the transceiver, but not out of the coupler. Plan is to measure the transmitted power at approximately 1 km with a calibrated antenna ("known gain").

Also, after tuning to a test frequency, measure the received power at the coupler's transceiver connection, using a temp system transmitting from location approx. 1 km away. Also planning to measure the power from the temp setup with the calibrated antenna at the same distance from the temp site, as the installed antenna, but not near the installed antenna.

I believe this should be ground wave propagation for these frequencies. Trying to verify the reciprocity of the antenna and compare to the vendors estimated gain.

 

[VEBill]

Is it a simple (straight) monopole? Does it have any loading coils or top hats?

A lambda/4 monopole should be about 117 feet tall at 2MHz and about 78 feet tall at 3MHz. The rule of thumb for monopoles is feet=234/MHz (it is an approximation).

The peak gain will almost certainly be negative at least several dBi, possibly negative quite a few dBi. It will probably be quite a bit worse at 2MHz than 3MHz (a relatively huge delta frequency).

The peak gain is unlikely to be directed along the horizon unless the ground plane is superconducting. The peak gain is likely to be aimed quite a bit above the horizon, say 30 degrees plus or minus.

As you may know, 'short' antennas have low, or very low, input Z, so resistive losses become excessive. Use maximum power (I'm assuming in the kw class) for a while and then (after turning off the RF) look for over-heated components. Well designed 'short' antennas have insanely large conductors between the matching network and the feedpoint.

Also, ground (dirt) losses are typically a large part of any missing gain for such systems. It is not uncommon to have 120 (quantity) ground radials to try to provide a good ground plane.

Testing antenna reciprocity is more a test of your measurements than a test of reciprocity.

These things can be modeled (NEC), but since the ground (dirt) plays such a large role, is typically unknown, may vary over the site, and is poorly modeled (relative to conductors), it is worth thinking about what accuracy you expect or need before you even bother starting.

If it is a single installation that isn't up to snuff, then it might be much cheaper to simply install a 100-foot monopole (guyed mast) than to bother undertaking all these measurements.