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Author Topic: Correct procedure for tuning BugCatcher system  (Read 427 times)
K1DRW
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Posts: 15




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« on: February 22, 2007, 01:30:41 PM »

I am about to install a BugCatcher style of mobile
antenna system (2006 Dodge Grand Caravan, right-rear
hitch mount, 5' mast, coil, 4' whip, 20m thru 80m) and
would like a critique of what I believe to be the
correct procedure for tuning the BugCatcher:

Beginning with 75m:
  1  Install a coil-tap for 75m.
  2  Find resonance with MFJ analyzer (X=0).
  3.  Adjust coil-tap and whip to move resonance
closer to desired 75m frequency (3.800mhz)
  4.  Determine feedpoint impedance with MFJ analyzer.
  5.  Install variable tap UnUn to impedance match  50ohm feedline.
  6.  Check for acceptable SWR at 3.800mhz.
  7.  If SWR NOT acceptable..use ATU..?
Continue with 60m:
  8.  Install next coil-tap for 60m.
  9.  Repeat steps 2 and 3, for 60m.
 10.  At this point I am not certain how to proceed.
I hope I don't have to change the UnUn matching for
each band ?
 11.  continue with 40m then 20m.

A critique of this procedure with constructive
comments would be sincerely appreciated.
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K1DRW
Member

Posts: 15




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« Reply #1 on: February 22, 2007, 01:34:47 PM »

Correction:  left-rear hitch mount
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K0BG
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Posts: 9883


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« Reply #2 on: February 22, 2007, 01:49:39 PM »

If you use an UNUN, you may very well have to change the taps. Although not deal, you can wind a shunt coil for the base of the antenna, and get the SWR within reason (<1.6:1) on 8 through 10. The coil needs to be about 1 uH. I use one that is 1.25 inches in diameter, 8 turns, # 14 Thermalese. The inductance can be changed by stretching or collapsing the windings.

You didn't say what sized coil you were going to use.  If you use too large of a coil, say 6 or 8 inch, there will be a difference in performance from 17 up. A good compromise is a 480 from TBC.

Alan, KØBG
www.k0bg.com
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K1DRW
Member

Posts: 15




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« Reply #3 on: February 22, 2007, 04:47:43 PM »

Alan:
   How do I determine when to use a shunt coil -vs- a UnUn..?
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K0BG
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Posts: 9883


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« Reply #4 on: February 22, 2007, 06:31:17 PM »

In some applications, where excessive stray capacitance is present, or you use different loading coils (mono band antennas), an UNUN makes better sense. In you case, if you have to stop to change taps, the UNUN may be the better choice. However, with remote controlled antennas (HiQ, screwdrivers, etc.), the base coil is your best bet.

Alan, KØBG
www.k0bg.com
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