jhs%Mitre-Bedford@sri-unix.UUCP (09/11/84)
In my experience, antenna tuners are generally a waste of time for receiving. If the receiver is any good, the limiting factor is usually noise and interference external to the receiver. The tuner affects the desired signal and the undesired interfence equally, and does nothing to help you discriminate between them. If your receiver is so poor that you are limited by received signal strength and receiver front-end noise, you'd do better to get a preamp. For most receivers, this will be of significant value only on frequencies above about 15 MHz, but if you do a lot of listening up there, it might be worth it. 73, John Sangster, W3IKG jhs at mitre-bedford
sullivan@acf4.UUCP (09/14/84)
Tuners no good for receivers? Ah, this is not so! My ICOM R71A manual says that "for best performance" one should have an antenna tuner to ensure that the receiver sees 50 ohms (or as close as possible). And sure enough, an antenna tuner made a big difference in signal catching capability. I should add that I don't believe that my "receiver is so poor that [it is] limited by received signal strength and receiver front-end noise." In any case, I no longer use an antenna tuner since I now have a McKay-Dymek DA 100D active antenna. (I couldn't put up an antenna in my new apartment, so I bought this and sat it on a window sill). The DA 100D presents a nice load to the ICOM, and works out better than my previous antenna tuner/fire- escape antenna. I recommend this antenna to anyone who is in a similar predicament. David J. Sullivan, WA1TNS sullivan@nyu-cmcl2 New York University ...!ihnp4!cmcl2!sullivan
jhs%Mitre-Bedford@sri-unix.UUCP (09/21/84)
I guess I am coming around to the view that if your receiver has a 50-Ohm input and that's it, brother, then a tuner would be useful. A preamp or an "active antenna" might be even more useful. My comment was based on my experience with older receivers which usually have an "antenna trimmer" or "preselector tuning" knob, which does one of the more important functions of a tuner, namely tuning out the reactance of the antenna so that it doesn't detune the receiver front end. Also, many of these receivers give you a couple of choices of input impedance such as a "short antenna" input as well as maybe 75-Ohm unbalanced and 300-Ohm balanced inputs. With this much flexibility, a separate tuner usually isn't much help. If you're stuck with a fixed 50-ohm input jack, that's different. If you REALLY want to hear some signals, try putting up a nice log-periodic rotary beam! 73, de W3IKG
Taylor.WBST@XEROX.ARPA (09/26/84)
There's a distinction which should be considered when choosing an antenna tuning/matching configuration-- especially when the antenna is small (ie: less than 1/4 wavelength). If the antenna proper is adjusted to resonance first and then a matching network is adjusted one may have improved signal reception -- as much as 5 to 10 decibels improvement! The reason for this may seem a little subtle. If the antenna is resonated first we can control by the method of resonating just where the current and voltage loops are along the length of the radiator independently of the subsequent matching process. For example, by this procedure one can locate a current loop at a high point on the radiator. However, if one just peaks the antenna trimmer in the receiver or, alternatively, peaks an external antenna tuning unit the current loop may be located at an undesired point, perhaps deep down in a coil inside the case of the tuner-- not a good spot to a intercept incoming radiation! As a practical case, I can get about 6 db higher signal-to-noise at 4 mhz using a 30 foot long loop of wire by series resonating this, using a noise bridge for indication, as compared with just using the antenna trimmer adjustmaent in my Uniden SR-2021! The latter case picks up much more electrical noise since only the end of the radiator is exposed-- it makes a superior electrostatic probe! Good luck! Jim (W2OZH)
jhs%Mitre-Bedford@sri-unix.UUCP (09/27/84)
An interesting comment, that - it got me thinking! I had always assumed you cannot move the current distribution in an antenna by fiddling with a tuner at the feed point. For antennas terminating out there in an infinite impedance (the insulator at the end), this is indeed true: the current is always zero at the end and always has its maximum a quarter wavelength in from the end. No matter what you may do with a tuner in your shack. Or even a tuner up at the feed point! However, you have educated me to the fact that if you bring the end of the antenna back into the shack, as with a loop antenna, you can indeed hang non-infinite impedances on its end, and can therefore change the standing wave pattern. Certainly you can short the end, getting a current maximum at the shorted point. You can also leave it open, getting a current zero there as with the end fed antenna. Presumably you can hang other (complex) impedances there and put the current maximum anywhere you like. Neat. In particular, I agree that for a large loop (i.e. non-infinitesimal) this should make it possible to place a current maximum near the highest point on the loop. By reciprocity arguments one can see that this should indeed make the receiving loop perform better than it would otherwise. However, I still claim that for the "good ole" receivers at least, and at the lower HF frequencies, tuning the tuner will move the S-meter higher, but with a decent AGC the audio won't sound any different. I guess they don't build AGC circuits like they used to. (Gotta keep this controversy going somehow!) Somewhere between 20 Meters or so and 2 Meters, the SNR limit moves from external noise to front-end noise, and then I agree that the tuner is important. Of course it might be true that with the loop, changes in the directionality of the loop due to current distribution changes of the type conceded above might affect the signal to external noise ratio. Thanks for educating me to a phenomenon I was unaware of! -73, John Sangster, W3IKG