RadioBanter - View Single Post - Determining if reactance is capacitive or inductive.

[email protected] · December 20th 06, 02:16 PM posted to rec.radio.amateur.antenna

wrote:

What concept am I screwing up here?

-Scott, WU2X

Here are the values I measured:

Freq R X SWR
--------------------------------------
144 44 2 1.1
144.25 46 4 1.1
144.5 55 7 1.1
144.75 60 7 1.2
145 68 0 1.3
145.25 78 0 1.4
145.5 79 0 1.4
145.75 73 11 1.5
146 63 21 1.5
146.25 53 23 1.5
146.5 46 21 1.5
146.75 40 18 1.5
147 37 10 1.5
147.25 35 8 1.4
147.5 37 6 1.3
147.75 41 10 1.3
148 48 15 1.3
148.25 59 19 1.4
148.5 75 16 1.6
148.75 90 0 1.7
149 96 6 1.9

Hi Scott, It probably means you are measuring the impedance through a
length of lossy coax. If you do the sweep using an electrical 1/2
wavelength of coax at say 146 MHZ you will probably see more of what
you would expect. I think the MFJ manual gives instructions for cutting
a 1/2 wave xmission line. If you have EZNEC you could do an SWR sweep
of your particular antenna and see what the impedances should be. Then
take the MFJ readings right at the antenna or through a multiple of 1/2
wave coax. The results should pretty closely agree.

If you still have problems, then there may be some strong RF present
that is disturbing the measurments, or YOUR presense in the near field
of the antenna could be upsetting things. If you are trying to measure
a voltage fed antenna like a J pole, if you are physically close to the
antenna it will foul things up. Good Luck.

Gary N4AST