[TowerTalk] 2 Meter Balun Question

Jim Brown jim at audiosystemsgroup.com
Thu Nov 21 12:48:52 EST 2013

On 11/21/2013 5:19 AM, Patrick Greenlee wrote:
> At a height of 50 ft or so it worked quite well on both bands. The 
> coax ran inside the aluminum mast.  No ferrites were used.

As N6BT has long observed, everything "works," even a light bulb as an 
antenna. A primary purpose of a common mode choke on a feedline is to 
minimize the coupling of noise from the feedline to the antenna.

As to using the marine-band antenna on 2M -- it depends entirely on the 
design of that antenna (its impedance vs frequency), the length of the 
feedline, and the output stage of your transmitter. If the mismatch is 
large enough, the output stage will provide less power, both because it 
is mismatched, and also because it "folds back" (reduces output to 
protect itself). The mismatch will add a bit of loss to the feedline, 
but that won't be a lot if you're using big coax like RG8/RG213 -- the 
line isn't long enough for this to be a big deal unless the SWR is 
greater than about 4:1.

The best answer is to get the data sheet for the antenna and study the 
graph of SWR vs frequency.  Or get one of these antennas and measure the 
SWR with a decent meter. FWIW, 10% bandwidth is fairly wide, but two of 
our ham bands are that wide or more.  160M is 10%, 80/75 is nearly 14%, 
and lots of us are able to match our wire antennas to our rigs and work 
a lot of DX.

73, Jim K9YC

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