[K4RY] AUARC: 80 meter dipole antenna results

[wb4lnm at aol.com]

Here's a bit more information on the 80 meter dipole antenna system that I promised earlier today.


I call it a 'system' because it is really the dipole plus the transformation properties of the very long piece of transmission line we have feeding it.  While I was making measurements in the shack, it needs to be realized that what I was really measuring was the input impedance of the dipole AND the impedance transformation actions of the long feedline.


Anyhow, it appeared that the dipole is cut just a bit long, which is not surprising.  Depending on the instrument that I used, the least reflected power was in the region of 3685 to 3725 kilohertz.  If we plug this into the rule of thumb 468/fMhz equation, we see the antenna is effectively about 126 feet long right now.  One of the more useful points for resonance is more around 3950 kHz, since the Alabama voice nets are on 3965, Mississippi on 3935 and Georgia on 3975 kHz.  Plugging 3.95 MHz in, we get a needed length of 118 feet, or about 8 feet shorter than the antenna is right now.  So, taking about 4 feet off each end would be my suggested 'pruning' to bring it closer to a useful section of the band.


That said, I must say after listening to all of the noise from switching power supplies that operation on 75/80 meters may be quite limited.  I seriously doubt if we will make a dent on all of the possible switching power supplies in Broun Hall!  However, looking for one or two of the worst offenders might be a good club exercise in hunting down RFI causes, and perhaps we can minimize a few of the worst offenders.   Fortunately, after about 1830 local time, ionospheric absorption really decreases and regional signals get quite loud, and many voice signals will overcome that noise.  It appears that some of the equipment goes off at night as well.  So, the efforts of shortening the antenna and moving the resonance point to 3950 will not be in vain.


With the resonance point adjusted, then comes the matter of impedance matching.  What I saw with some limited instrumentation was a best case of about 4:1 VSWR.  The solid state radio will not 'like' that and will automatically cut the output power to minimize the possibility of damage to the final power amplifier transistors.  The tube radio is a bit more forgiving, but would need some help, too.  We do have the TenTec 238 L network tuner(*), and I had little problem hooking it up to either radio and getting 100 watts out *over a limited band of frequencies*.  Basically, one must stay within the impedance matching range of the device.    For the short term that's probably an effective approach.  Longer term, we may want to figure out a fixed or limited range network to just leave on that antenna all of the time.  One of the characteristics of a sharply bent antenna as we have it currently erected is narrow bandwidth.  


That's the report from Monday afternoon.  73  John...  WB4LNM


(*)  The TenTec AMU has a worn-out potentiometer as part of the VSWR bridge that makes it very difficult to tune in a well-matched system.  I'm going to be looking for a copy of the manual to find the value for that so we can get it replaced.  Also, the tuning knob for the capacitor has nothing but a friction fitting, and it falls off when trying to use it.  Some strong glue will most likely take care of that problem.    jhk






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