The MFJ259 doesn't give you the resistive part of the impedance when the
SWR is not equal to 1:1. From the tests I have done with it , I think
it is giving you the total impedance. In the mnual, they imply that at
minimum SWR reading, you can read the resistance, but this is not true,
if that SWR is far away from 1:1.
As you measured, you can have a 3:1 SWR and read 40 ohms on the meter.
This says that the equivalent impedance of the circuit being measured is
40 ohms. It doesn't tell you anything about the actual resistance or
reactance of the load. The SWR is defined in term of reflection
coefficients, not in terms of resistance ratios, or in terms of
equivalent impedance ratios. In your case, you have a circuit with high
reflection coefficients (caused by high reactance).
Also when the meter reads minimum SWR, this is not necessarily the exact
resonant point. Minimum SWR doesn't always coincide with zero
reactance. It is usually close but not exact.
When I was using the MFJ259 and needed to know the actual R+jX, I used
to take a variable cap or a variable inductor and put in in series with
the load being measured, set the MFJ to the frequency I wanted, tune the
cap or inductor for minimum R. This will tune out the reactance of the
load and let you measure the actual resistive part at this frequency.
To get the reactive part, I would measure the cap/inductor on a bridge
and calculate the reactance of the load. A lot more trouble than using
the MFJ259B, but if it's all you got...
Tom Osborne wrote:
> Hi all
>I just put up a 160 meter inverted L and the antenna is resonant after
>tweaking about 1850. It shows a resonant SWR of almost 3 to 1, but on the
>MFJ 259, is shows a resistance of around 40 ohms at the resonant point. I
>have about 70 feet vertical and the rest horizontal. The only thing is
>that up in the tree, it is about 6 feet from the 80 meter inverted V.
>Would that make it so bad?
>This antenna has LOTS of radials. I also have 3 40 meter verticals, and I
>tied the 3 of the radials from the 160 inverted L to the hub each of the 40
>meter radials, so there is probably 3/4 mile or more of radials.
>I know that it is not good to have a perfect 1:1 SWR on a vertical if it is
>playing right. With the 40 ohms of resistance, it seems as though it should
>be 1:5:1 or so. I could live with that---but 3:1? What's happening. 73
>See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
>Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
>questions and ask for Sherman, W2FLA.
>TowerTalk mailing list
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
TowerTalk mailing list