[TowerTalk] Tuning Antennas with the MFJ-259

[Jim Brown]

On Sat, 28 Mar 2009 23:59:49 -0500, AD5VJ  Bob wrote:

>Well one of the antennas I will be working on in the morning is 
>going to be an 80 Meter dipole, but I can only get it up here at 
>abt 30' then I will be taking it and putting it up at around 60 
feet, so it sounds like I want to tune it high 

Bob,

You will see SOME difference on 80M between 30 ft and 60 ft height, 
but NOT a lot. If you tune the antenna at 30 ft, the resonance will 
move up a bit when you raise it to 60 ft. I'd guess 1% or so. In 
other words, if you want the antenna to resonate on 3600 kHz, tune 
it to 3560 kHz at 30 ft. Lowering the ends (inverted V style) also 
lowers the resonant frequency a bit. I would expect the SWR curve of 
an inverted V with the ends at 30 ft and the center at 60 ft to be 
not very different from a flat dipole at 30 ft. 

The other thing that happens is that the IMPEDANCE at resonance 
rises as the antenna is higher. The impedance of a dipole at 
resonance is close to 72 ohms in free space, but gets lower when it 
gets close to the earth. 75 ohm coax is the best match to a HIGH 
dipole, while 50 ohm coax is the best match to a LOW dipole. 

73,

Jim Brown K9YC