Antenna Measurements

[rvargo]

Hi,

I have a question on measuring gain/efficiency. We use antennas in a plastic housing, and we would like to see how different changes affect the gain/efficiency of the antenna.

Currently all we have is an Agilent 8714ET 2-port Vector Network Analyzer, which we use to measure the VSWR. Is there a way to use the 8714ET to measure gain/efficiency? What other equipment would we need to get?

Any suggestions would be appreciated.

Thanks.

 

[Higgler]

Normally, after recording VSWR, if you need more, you record antenna patterns in an anechoic chamber over as much of a sphere as you can, then mathematically integrate the antenna pattern shape to calculate the peak directivity (which is based on antenna pattern shape only).

In the original calibration, you'd have calibrated with a known gain antenna. One normally compares the peak directivity calculated to the peak gain measured (corrected relative to that known gain standard) as a measure of efficiency. Most efficiencies of antennas are in the 70-95% band, and it's more commonly shown for very narrow band antennas than for widebandwidth antennas.

Also, google "wheeler cap method". It's a method to evaluate antenna efficiency in one simple VSWR measurement, but you need to enclose everything in a metallic cap.

For quickie pattern comparison, you can use the 8714ET and make an S21 measurement between a probe antenna (make it or buy it) and your antenna, spin the unit under test using a lazy susan and foam (to space your unit off the lazy susan), and manually record S21 for one unit. Then change your unit/materials and repeat the test with everything positioned exactly the same (UUT position, cable routings, probe antenna, etc.). Your probe can be very close to your unit under test and still get a reasonable comparison.

If you want to see which plastic is lossier, put them in a microwave over separately and see which heats quicker. That'll give some feel for loss. Of course, don't put metal in the microwave.

kch

 

[zappedagain]

I always put a mug of water in the microwave too along with my test sample so something is absorbing the microwave energy if the test sample is not lossy. Otherwise I've heard that some microwaves will shut down when they sense the power is not being absorbed so this can give a false reading. And once the water boils I know I've run the test for a reasonable amount of time!

 

[Dunning]

Another antenna question ...

I am working on a project to set up a low-power AM transmitter under USFCC rule 15.219. This contemplates operation in the AM standard band, theoretically from ca. 530 mHz to 1700 kHz. Maximum permitted rf power = 100 mW or 1/10 watt.

However, the rule also specifically limits antenna length to 3 meters. This is apparently a deliberate "Catch-22" to further limit actual radiation and hence broadcast coverage.

The antenna (oh, yes, the FCC also makes it difficult to build your own equipment, kit or scratch, by imposing rather stringent proofs on the maker, so there are two or three manufacturers in the field; the rules also say the antenna supplied by the manufacturer must not be modified) is a whip about 102 or 103 inches, just about 2.9... m. long.

I have suggested to a couple of manufacturers that a capacitance hat might increase radiation efficiency without breaking the 3-meter limitation -- even if the folded length has to be included in antenna length. The hat would only be 8 or 10 inches, not enough to break the rule, even though it isn't really "longer" if the hat is folded back. I won't argue with the rules guys, though -- just add the length of the capacitance hat fingers to the total.

So here's my question: under these circumstances -- flea power, upper end of the AM broadcast spectrum, say 1300-1700 kHz, vertical radiator (probably not more than 20 feet height from ground-level to the center of radiation -- is a top hat going to make a significant difference in the efficiency, and hence coverage radius and area (which is on the order of 1 mile radius or less anyway)?

It goes without saying that a good ground is implicit here: radials, ground rods, and all the usual suspects; 100% modulation is also a given, along with reasonably careful antenna tuning, insofar as the short antenna allows. I'm looking for a little edge, trying to squeeze everything I can out of that QRP 100 mW. Thanks for your thoughts....

Bill Dunning