Circle On Reader Service Card

chances of uneven curing and crocking due to rapid heat buildup, Ne\t. 1 poured the resin and allowed i[ to curc overnight. The curing site should be outside to eliminate any problems with the smelly fumes,

The next day. I separated the cast lenses from their molds. The molds .survived and could be used again. The edges of the lenses were rough and chipped, so I decided to trim or "edge" the lenses. I used a cylinder with an outside diameter of 5 inches—the hood from an old oscil-loscope—to mark the lenses. (See Figure 3.1 i he actual edging was done with a disk sander and only took a couple of minutes for each lens. After the edging, the next lask was to mount the lenses. Normally, lenses are mounted in a finely-machined barrel with a threaded retaining ring, I can operate a lathe and chase threads, hut let's be reasonable! Instead. I epoxied a 5V4-inch-square piece of '¿ inch acrylic 10 the Hat surface of each of the epox> lenses. r$ee Figure 4 ) Then I drilled four mounting holes in each acrylic square.

Light sanding of each lens, followed by a couple of coats of clear acrylic spra>, finished [he lenses. The sanding is for two reasons. First, although the lenses arc of poor optical quality , they can still produce a very hot spot when aimed at the sun, unless their surfaces receive a ground scattering finish. Second, they look better with an even. IVosted appearance.

Testing the Lens

One nice thing about small aperture microwave antennas is thai you can set upa test range in a short distance. For example, with a small horn source antenna and the cast lenses, a distance of 20 feet is more than adequate. For a source, I used j surplus 2K25 klystron that had its cavity stretched to operate on 10 4 GHz. (See "A Complete X-Band Transmitter" b> Stirling M. Otberg W1SNN, 73, August 1978.) People may shun vacuum tubus, hut I can only respect a rube that, like me. is over 40 years old and still working. Besides, have you priced a Gunn diode lately? I would very much like to hear from anyone with a supply of Solían Gunn diode units, Anyway, the 2K25 puts out about 25mW when used with an old surplus klystron power supply thai ] got at a ham-lest many years ago. The receiving portion of my antenna range is a 0-40 dB attenuator and a tunable detector mount. For metering, I now use a circuit published b> Chuck Houghton WB61GP. (See "Microwave Test Equipment for 10 GHzf 1 by Chuck L. HoughtonWB6IGP, 731October 19KH) [did

Ph oto A. Or i gbtal jss t ens and p laste rmok i.

Photo B Cast lens epoxied to acrylic plate and lens mounting pi me.

Photo C Mounted lens, mounted httrn and complete antenna.

have to reverse the polarity of the detector diode to get it io work, probably due to a misprint. His circuit is very compact and very sensitive. You cannot go wrong reading his mail} articles, and he now has a superb column, 14Above and Beyond," in 73 every month,

I placed a standard gain horn on both the transmitter source and the receiving setup, and tuned the detector mount for maximum response. Then I set the S-meter to mid scale by adjusting the variable attenuator. The attenuator sett i ng at this point became my reference reading Substituting other antennas on the receiver setup, the attenuator is changed to maintain a midscale meter deflection. The reference reading is subtracted from the new setting. The result is added to the gain of the reference antenna io yield the gain of the new antenna.

Sound more complicated than it actually is? Saj you have a reference antenna of 16 dB gain and with it you get a reference attenuator setting (reference reading for midscale meter deflection) of 8.5 dB. Putting your test antenna on the receiving setup, you get a midscale meter deflection with the attenuator changed to a setting of 12 dB. Then, subtracting the reference reading (12 - 8,5 = 3 5 dB), you find that the test antenna has a gain 3,5 dB greater than the reference. Vou have to attenuate the signal from the test antenna to make it equal to the reference signal. The gain of the test antenna is the difference added to the gain of the reference antenna (16 + 3.5 = 19,5 dB), if the midscale deflection had required an attenuator setting of 4 dB for the test antenna, then subtracting the reference reading (48.5 = -4.5 dB) tetis you that the test antenna has a gain of 4,5 dB less than the reference, or a net eain of

You do not need to know the absolute gain of an antenna to use this technique. As long as you use the same antenna as a reference, you can measure ihc relative gain of your other antennas to it. After all, we are more interested itl reducing losses or improving gains than we are in knowing what the absolute levels are

Back to the lenses. The lenses, having a focal ratio (or I of about I, should be fed with some sort of small horn antenna, t used the only one I had at the time—a Microwave Associates metalized plastic 17 dB gain horn. The addition of [he epox> lens increased the gain by 4,5 dB* or a factor of 2.8. With a unit at each end of a path, the net s\stem gain is 9 dB, allowing the usable separation to be increased by 2.8 for equal signal strength. 1 uas abie to mount the lenses on one lace of a inch cube steel utility box and mount the horn with a spacer on the opposite face of the box, The result is a compact, easy-to-handle unit that can he readily mounted on a tripod for field work, or on a lower for point-to-point applications. (See Figure 5.1

i am nou looking for a larger lens to cast, or perhaps I can find a large bow l to serve as a moid. HQ

Contact John M. Fnmke WA4WDL at 23 Parkwood Drive, Apt, 20!. Yorktown VA

23m,

0 0

Post a comment