[K4RY] AUARC: 80 meter dipole antenna results
John Y. Hung
hungjoh at auburn.edu
Wed Apr 6 14:35:04 CDT 2011
We might consider replacing the tuning knob with one that has a set-
screw in it. I will take a look next week when I get back.
The necessary impedance match network will be an interesting project
-- makes our Emag/Smith Chart studies come alive, folks!
John Hung
On Apr 6, 2011, at 12:28 AM, wb4lnm at aol.com wrote:
> 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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