[K4RY] AUARC: 80 meter dipole antenna results

[John Y. Hung]

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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