by Alan Dewey WB9JTK

Technical Specialist ARRL South Florida Section

The first RF field strength measurements were to take place at the Boca Raton Amateur Radio Association "Free Flea" swapmeet February 25. We showd up to a constant drizzle and overcast skies. With the poor outlook for weather, many attendees left by 09:00. The rain stopped approximately 09:30 which allowed us to begin setup. Thanks to N8JUC for a Honda EU2000 generator. This generator is extremely quiet acoustics-wise. Also, no RF emissions were seen from this generator.

We did not intend to measure antenna 'efficiency' directly. That would be very difficult, time consuming, and still would not be accurate. We did make accurate relative measurements of the RF that was bing radiated toward the horizon. Please keep in mind that this energy is not what makes DX contacts. The measurement antennas were placed 1 meter above the earth. The purpose is to give us all a chance to see how our antennas compare. For example we read articles telling us to never mount your HF antenna on the bumper: but how much difference does that really make ? How much real difference is there between a Hustler whip and a screwdriver antenna?

On 2 meters the equipment for these meausurements included HP141T, HP8554B, 8552B spectrum analyzer. The measurement antenna was an Electro-Metrics BIA25/30C biconical dipole. The HF measurements were made with an 8553B spectrum analyzer and an Electro-Metrics ALR25M magnetic loop. The measurement antennas were mounted on a wooden tripod so that the lowest edge of the receiving element was 100 cm above the earth. In worst cases readings were made to within 1 dB.

K4THL used his handheld and tried 4 different 'rubbery duck' antennas. Because body position will affect the field strength by 15 dB or more, Tom held the handheld on top of the bed rail of my truck. This kept the readings constant even with changes in body position.

The 17 inch whip outperformed the 1 inch tall MFJ by 23 dB ! (Which means it is 200 times more efficient.) However, I will continue to use that little 1 inch MFJ antenna when appropriate, such as at a hamfest, where I am only going to be talking a few hundred meters.

Then next set of tests were performed to see if there is significant directionality from a 2m whip on a car. N8JUC parked his car in 3 different orientations and field strength was measured. (Distance was not measured.)

On 17 meter band we had the chance to measure two mobile installations and 1 portable antenna system. The mobiles were WB9JTK with a 15m Hustler operating on 17m band by using an excessively long 'stinger'. N4TZH arrived with a magnetic mount Buddipole on the 17m band. KF2GQ set up a dual Buddipole arrangement for horizontal polarization.

N4TZH operated with 5 watts output, and WB9JTK used a 100 watt radio. After correcting the readings for the 13 dB difference in transmitter power, we determined that the Buddipole outperformed the Hustler by approximately 1 dB.

Because KF2GQ was the only antenna horizontally polarized, and the only antenna with significant height, we are not able to publish any differences in the field strength measurements.

The two portable antenna systems were brought by NE4LS and KF2GQ. The NE4LS antenna was a 1/2 wavelength long end-fed wire supported by a 33 foot telescoping MFJ mast. KF2GQ used the same Buddipole dipole as on 17m, but re-tuned for 20m. We did have time and opportunity that NE4LS 'rotated' his antenna system to see if exhibited directionallity. There was only one mobile station available to operate on 20m for comparison.

In the case of the portable antennas, NE4LS was able to determine that his end-fed halfwave (mostly vertical) did exhibit 3.5 dB front-to-back. After correcting for power differences (Nelson was operating approximately 650 milliwatt) we find that in the favored direction, his push-up pole halfwave outperformed the Hustler whip on WB9JTK's truck by over 5 dB.

Again, KF2GQ buddipole was the only horizontally polarized antenna present. We did not have the opportunity to rotate the dipole to find the maximum field strength.

For antennas with identical efficiencies, the theoretical difference between 17m and 20m will be 2.2 dB. (20 * log (f2/f1) Comparing isotropic radiators with identical power input, the field strength measured on 17m band will be 2.2 dB weaker than that measured on 20m band.) This allows us to compare our antenna on one band to another.

What we expect to see in reality, using 'small' antennas, however, is that the field strength we produce is weaker on the lower frequencies, not stronger. This is due to the much-lower efficiencies with the increase in wavelength.

In the case of the Hustler whips on my truck we measured that the 20m signal was 5 dB weaker on 20m. If the antenna had the same efficiency on both bands, the signal would have been 2 dB stronger instead.

Comparing the buddipole dipole on the two bands, it was determined that the field strength was 8 dB lower on 20m than 17m.

This plot illustrates why we can not compare the buddipole dipole to the mobile whips. The field strength measurement antenna was 1m above the ground. Imagine if the measurement was made right on the horizontal line in the plot above. Note that a very small change in angle will make a very large change in the field strength measured.

This plot was generated by EZnec. The blue trace is a shortened dipole 15 feet above poor ground. The black trace is a 7 ft tall whip on poor ground. Neither of these were accurate representations of the buddipole dipole nor the hustler mobile whip. I have included this plot merely to illustrate that we cannot compare these two types of antennas based on any information gathered under the circumstances we used. Note that at very low angle (along the horizon) the mobile whip (black line) is stronger than the dipole (blue line). But at an angle of 15 degrees the dipole beats the whip by several dB.

We hope to do this again very soon. We invite anyone that has access to a good location to contact me with your suggestion. Also, with more advance planning we could perform more educational tests. It would be best if hams would organize with others to bring antennas that are similar. I have a personal interest in comparing mobile antennas. Perhaps we have a few more backpackers out there that would like to compare more 'field antennas'. I have the ability to compare RF field strengths down to 0.2 dB difference under controlled circumstances.