Gain calculation based on measurements of recovered signal of known power:

4TTR /Pr

X V Pt

G= total gain of two indentical antennas under test

R= separation of antennas under test, in units of X

X= wavelength Pt = transmitter power to one antenna (mea­

sured at the feedpoint) Pr = received power measured at the feed-

point of the second antenna

In the case of the F9FT 55-element 1296 MHz Yagi, our test setup derived the following:

R = 75 feet = 100 X = 2300 cm X= 23 cm Pt = 5.00 watts Pr = 22.4 mw

G = 1256.64 N/"04 = 265.98 (both antennas) Gain for one antenna = 265.98 + 2 = 132.99 Gain in dB = 10 log 10 132.99 = 21.24 dB

Fig. 1- Gain calculation formula.

identical antennas. The gain of either antenna should be exactly half the total gain measured, or 3 dB less. Pete and I set up our "antenna range" using two 10 foot masts supported by 3 foot tripods, spaced so that the most forward directors of the two antennas were 100 wave­lengths (about 75 feet) apart. This was a "far-field" distance, I reasoned, and the field was certainly unobstructed and quite free of re­flecting objects. To assure the antennas would be in the same horizontal plane, we used a very level section of pavement (auto road) aside my house as the "range." We did get some unusual looks from the occasional passerby, but proceeded in the footsteps of Montgomery (not to mention Maxim, Tilton, et at) to make our measurements.

We mounted a Bird Model 43 Thruline coup­ler within a few inches of the feedpoint of the "transmit" antenna, and installed a 1.2-1.8 GHz element for measurement of Pt, transmit power. We mounted a microwattmeter RF de­tector (Boonton Electronics type 41-4D power detector good to 12 GHz) within a few inches of the feedpoint of the "receive" antenna for measurement of Pr, recovered power. Rather than assemble long DC cables for the meters themselves, we used the standard-length cables and supported the Bird power meter and Boonton microwattmeter from the anten­na booms using stretch cords. The meters hung sufficiently below the antenna booms to be out of the plane of radiation. Despite their hanging nearly 10 feet above ground, the meter scales were easy to read, since they are large and we had plenty of available light.

Using an SSB Electronics LT23S 1296 MHz transverter driven by a Kenwood TR9000 mul-timode 2 meter rig, we generated enough power to indicate exactly 5.0 watts at the transmit antenna's feedpoint. It should be noted that there was no measurable reflected power. This "exact" 5 watt measurement should be ± 5 % , as previously confirmed in a laboratory, using precision equipment (micro­wave bolometer and 0.1 dB accuracy power attenuators). The Boonton microwattmeter, set to the +10 dBm range, went right off scale! Eureka! The doggone system works.

Climbing a small step ladder, I changed the microwattmeter range to the +20 dBm (100

mw) position. I then moved the ladder away, and gently twisted and tilted the receive an­tenna mast for a peak detected meter reading. After some fumbling around, I was able to achieve a stable + 13.5 dBm (22.4 mw) read­ing while Pete reconfirmed the transmit power to be 5 watts. This was all somewhat anticli-matic, in that our actual measurements took all of about two minutes, while the setup con­sumed a couple of hours.

I couldn't wait to plug the numbers in the an­tenna gain formula to see if our readings were "in the ballpark." Let's see . . . R = 2300 cm, Pt = 5w, Pr = .224 w. The formula yields an answer for total antenna gain, expressed mathematically, not in dB. The answer: 265.98. The gain for one antenna, then, should be 265.98/2, or 132.99. The gain in dB should be 10 log 132.99, or 21.24 dB.

How close is this to the gain claimed by the manufacturer? Very close. Tonna claims the gain of their 55-element 23 cm Yagi to be 21.25 dB! I don't think we could have corre­lated better than this.

We did not attempt to measure the antenna pattern, as this would have been a bit com­

plicated for our simple antenna range. Tonna does provide a polar plot of their antenna, taken in both the £ (vertical) and H (horizontal) planes, and the H-plane plot is reproduced in this article (see fig. 2). They state the 3 dB beamwidth to be "2 x 5.41 degrees," or 10.82 degrees. This is in line with the claimed (and measured) antenna gain, and shows the an­tenna to be very sharp—possibly too sharp for the average installation. I'm not a big fan of really sharp antennas for routine VHF work, as they are too critical to steer and make even scheduled contacts too easy to miss. How­ever, most 23 cm operators are becoming ex­perienced enough with sharp antennas, in­cluding large parabolic dish systems, that these obstacles can be overcome with pa­tience. Besides, the 55-element F9FT antenna appears to have a dozen sidelobes (not unus­ual for very long-boom arrays) which would serve to assist in making contacts with "locals" not peaked by the forward lobe. Ac­cording to the Tonna-supplied plots, the first six minor lobes on each side of the main are within 20 dB of the main lobe's intensity.

In all, I'm very satisfied that the Tonna

Fig. 2- H-plane polar plot, 55-element F9FT23cm Yagi.

ANTENNES TONKA 5.H. j 32 BouWyo'-d OoupMnot 51100 RElnS FRANCE

DIAGRAHtlE DE RAY0NNEHENT CALCULE: ANTENNE SS ELEHENTS LONG TAG]

FREQUENCE: 129S.8 nHz PLAN: H

GAIN CALCULE 21.2S dB ISO RAPPORT AU./ARR.: 24.72 dB

ANGLE D'OUUERTURE A -3dB : 2 x 5.41 dug.

64 • CQ • January 1986 Say You Saw It In CQ