rjr@mgweed.UUCP (Bob Roehrig) (09/18/84)
I can personally vouch for the fact that some swr indicators built into antenna tuners are "off"! A friend of mine has a MFJ tuner (not sure of the model) with a built-in bridge. When he adjusts the tuner for minimum reflected, the power out of his TS-430 is half of what it should be. Adjusting the tuner using a Bird wattmeter, everything is as it should be. The MFJ bridge then indicates about 1.8 to 1. I noticed the same thing with my old Heath swr bridge on my TS-930. As to someone's comment about not needing a tuner for receivers; This may be more true with older receivers with tuned input circuits such as Hallicrafters or Hammarlund or especially those with tunable preselectors such as Drake or the Yeasu FRG-7. However with receivers that have a fixed broadband front end, such as the Kenwood R-1000, etc. a tuner makes quite a difference, especially at lower frequencies. True, this may not affect the signal to noise ratio much, but I have seen cases where on 80 meters a tuner can make a difference of more than 10dB. These newer types of receivers want to see 50 ohms! I should further say that the tuner is especially useful when using some random length of wire for an antenna, not one that is resonant at the frequency used. Concerning tuners that have a balun to feed a balanced line; This is a compromise situation. Balunes are only good for a narrow impedance range and then only good if the load is not reactive. This is seldom the case with open wire fed antennas. The balun should be between the rig and the tuner where it sees a resistive load. So far as I know, Palomar is the only one that has used this approach. For feeding a balanced line, the best bet is still something like the old Johnson Matchbox, or one of the handbook circuits especially made for the purpose. Bob, K9EUI
parnass@ihu1h.UUCP (Bob Parnass, AJ9S) (09/19/84)
As for an antenna matcher not being needed on todays' SWL receivers: If the antenna matcher has the appropriate band pass/reject response, it will help keep strong broadcast band signals out of a broadband receiver on VLF and HF. -- =============================================================================== Bob Parnass, Bell Telephone Laboratories - ihnp4!ihu1h!parnass - (312)979-5414
jhs%Mitre-Bedford@sri-unix.UUCP (09/27/84)
Hurrah for the Johnson Matchbox. Does anybody know of one that is for sale? -de W3IKG