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Vintage(?) /\/\ Spectrum antennas

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Otis413

Otis413

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Anyone happen to have a "Spectrum™ by Motorola" (A88) 42-50 MHz antenna loading coil laying around? (P/n: 01-80358A88) I need to know how many turns the wire makes and what value the cap (if any) is. The inside threads out easily if it's not too corroded.
Why I ask: A while back I got some Spectrum antennas as a package deal with a few Maxtracs, three or four (A86) 30-36 MHz and two VHF (A90) 136-174 antennas. The VHF antennas where what I wanted, so the low band antennas spent the last few years in my parts box. Recently I was needing another VHF antenna and took apart one of the A86 coils and a A90 coil, they're the same inside, 'cept for the number of wire turns and the cap.
The low band A86 coils have 21.1 turns and a cap marked "160 +- 2% 500V", and the VHF A90 coils have 6-2/3 turns (spaced 1/8" apart) and no cap. I shortened the wire in the A86 coil and removed the cap, works purdy good for 2M.
Anyway, I'm building me another pickup, and have two Maxtrac radios to put in it (converted to 2M and 6M) I find my self in need of a 6M antenna, and since I already have the VHF Spectrum I'd like to convert one of the A86 antennas to A88 for 6M use, and that way have matching antennas...
I'v heard of people using 5/8 wave 2M antennas as a 1/4 wave 6M antenna, but that would end up being about 52" (or so) long, (I don't have a whip that long that will fit) and the A88 would only be about 36" @ 52 MHz.
Thanks for any help!
KD8VBV
 
Otis413

Otis413

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In case anyone besides me is curious, (I enjoy building my own antennas etc ) I ordered the 42-50 loading coil, along with a new base "gasket", with S&H it came to a little under $40.oo
Anyway, if anyone takes a notion to build their own base loaded 6m antenna, here's the design Moto uses:
8-7/8 turns of 14g coated copper wire with no gap between the coils wrapped on a 0.480" plastic core.
One 51pf 2% 500v mica cap at the base of the coil, to ground. (The 30-36 coil had a 16pf cap so I might experiment with increasing the cap on my 6m antenna a by a few pfs)
According to the 42-50 cut chart the whip should be about 39.5" @ 50 mhz (w/o spring) and 37.5" (w spring)
Looking at the chart it looks like the whip needs to be shortened -a little less then- 1.5" for each 1 mhz increase in frequency.
So I'm guessing about 37" @ 52 mhz (w/o spring) and about 34" (w spring). Of course, cut long and adjust from there...
Thanks for the link Jeepsandradios :)
 
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