View Single Post
  #16  
Old 02-18-2014, 01:36 PM
DaveInDenver's Avatar
DaveInDenver DaveInDenver is offline
Hard Core 4+
 
Join Date: Jun 2006
Location: Larimer County
Posts: 6,550
Default

OK, it'll tune but you have to very careful. I started with a whip that was slightly long on 2m and it was about 2:1 on the upper end of 70cm. For reference we get 144-148MHz on 2m and 420-450MHz on 70cm.

Starting point:
144 - 1.3:1
145 - 1.4:1
146 - 1.5:1
147 - 1.7:1
148 - 1.9:1

This gave:
420 - 2.8:1
425 - 2:1
430 - 1.3:1
435 - 1.5:1
440 - 1.6:1
445 - 1.3:1
450 - 1.6:1

I knew this was long and not ideal but didn't realize it was quite that bad. So I trimmed this antenna to minimize at 146MHz, which got it down to 1.2:1 at the middle of 2m. But this made 70cm go completely useless, 3:1 and worse throughout our band. It's perfect for 2m now, though! I actually knew this would be the case since the 2m and 70cm bands do not line-up as perfect multiples, so I figured if/when this happened I would either have a perfect 2m antenna or could always just cut it to a 1/4λ on 70cm.

Non-the-less, when you sweep a 3/4λ antenna it will not respond with a flat SWR curve as the various characteristics dominate. It's kinda like a 1/4λ sometimes but then it'll look a little like a 1/2λ or 5/8λ mixed in sometimes.

So you can't just pick a single frequency to dip and aim for it, you have to sweep across your bandwidth and pick a happy compromise. I let the lower part of 70cm do what it would since here in Colorado and most places the lower frequencies are SSTV, EME, SSB, weak signal and QRP. Since this is on my truck which transmit on FM for simplex and repeater phone only, I tuned from 433 to 450MHz, so let it bounce around 1.6:1 across there.

Trimmed my second:
144 - 1.2:1
148 - 1.6:1

This gave (approximately):
420 - 3.2:1
430 - 2:1
440 - 1.5:1
450 - 1.6:1
460 - 2:1
__________________
'91 Toyota Pickup
'09 Kawasaki KLR650
'12 Gunnar Rockhound 29

"They say the test of literary power is whether a man can write an inscription. I say, 'Can he name a kitten?'" -- Samuel Butler

Last edited by DaveInDenver; 02-18-2014 at 02:05 PM.
Reply With Quote