wjm@whuxl.UUCP (MITCHELL) (09/19/84)
There are several reasons why an antenna tuner can be benificial on an receiver as well as on a transmitter. Probably the most siginificant is the ability to improve selectivity throug another tuned stage - given the amount of QRM around these days (especially on 40, 20, & 15) this can be what you need to get that signal out of the noise. The impedance match won't hurt you either but that's not too critical on receiving (of course, that's why you want a tuner for your transmitter). 73's Bill Mitchell, WB2IAU, (whuxl!wjm)
danny@alice.UUCP (Dan Kahn, K1DK) (09/22/84)
I've never seen much reason to use an antenna tuner on a rcvr on HF, so I read with interest the articles on the net advocating the practice. Two reasons have been given, neither of which I find convincing: 1. "It will increase your S meter reading by an S unit or two." But in the great majority of cases on HF, if you are having trouble copying a signal it is because it is weak WITH RESPECT TO an adjacent signal or atmospheric or power-line noise, NOT because of ABSOLUTE weakness. Better antenna tuning increases signal AND interference/noise alike. (And if you're NOT having trouble copying the signal, who cares about the extra S units?) 2. "The antenna tuner is another tuned circuit, hence more selectivity, which should be helpful given all the QRM on 40 and 20 meters." (Approx quote.) But when people talk about the "QRM on 40 and 20" they are talking about QRM on the SAME FREQ or within a couple of KHz of the desired signal. No antenna tuner is going to help with that. (If you have a kilowatt xmtr a few hundred yards away, operating a few hundred KHz away from the signal you're trying to copy, the extra tuned ckt could help, but that is clearly a special case.) Dan Kahn, K1DK Bell Communications Research Murray Hill, NJ
mikey@trsvax.UUCP (09/24/84)
Oh oh!! Time for my two cents. (Is that really worth that much?) I have a book on radio theory from National. They go to some great lengths about the front end circuits for their broadcast radio circuits. They claim that there is a certain amount of extra gain that you can get by making the antenna appear as a capacitive load to the radio systems front end. I don't have the book in front of me but if anyone is interested, I'll reference the article and get the National book number. Back to the subject at hand, it applied in the article only to antennas on a large ground plane on receivers with inductively tuned RF sections (i.e. your standard detroit car radio) but I wonder how much of their principles would apply to the case of tuners on receivers? mikey at trsvax (KA5MJQ)
danny@alice.UUCP (Dan Kahn, K1DK) (09/27/84)
Re KA5MJQ's reference to technical lit on broadcast-band auto radio antenna matching and possible applicability to HF rcvrs: The front-end design in broadcast-band car radios is completely different from that in HF rcvrs. This is due to the fact that in a car you're using a 3-foot long antenna to rcv signals whose quarter-wavelength is several hundred feet. In a car radio, the capacitance of the antenna coax (coax mandatory for noise shielding) and of the antenna itself is part of the RF-stage tuned circuit; that large amount of fixed C is why the radio must be inductively tuned and why proper front-end tuning (i.e. setting trimmer cap) is critical. Dan, K1DK