On Sun, 21 Dec 2003 21:34:08 GMT, Dave Shrader
wrote:

|Let's go a little slowly here. It is not clear where the reading is
|taking place. He states 'if I put a cap across the coax'. Where is the
|reading? Across the coax or at the antenna?

The reading is where the coax connected to the MFJ thingie. Beats me
as to where that was. But I don't care whether the reading was at the
antenna, the middle of the coax or the transmitter end.

*Wherever* he took the reading is the *load* as far as I'm concerned
and that's where he needs to insert the -j35 series reactance to
cancel the +j35.

If he makes a measurement elsewhere and gets a different answer, then
he needs a different solution.

|
|He should make the reading at the antenna terminals and add compensation
|there so that the VSWR changes from 2.1:1 to approximately 1:1.

If the terminals are 100' in the air, in between two supports, that
might be difficult. But in principle I agree the best place to do the
matching is at the antenna. But when I say this, all of the guys
using ladder line with 50:1 SWRs get all upset.

|
|I don't have a problem with a 2.1:1 VSWR. I'd leave it alone or retrim
|the antenna.

Yep. The old Drake L-4B "antenna tuner" easily handles this.
|
|
|W1MCE
|+ + +
|
|Wes Stewart wrote:
|
| On 20 Dec 2003 23:00:17 GMT, (PDRUNEN) wrote:
|
| |Hi Group,
| |
| |A friend of mine let me borrow one of those MFJ antenna analizier with the
| |reads out of the R and X component.
| |
| |Now if I have a reading of 40 R and 35 X then if I put a cap across the coax
| |with an impedance of 35 ohm it should cancel out the inductive reactance and I
| |should only see the 40 ohm resistive?
|
| No. The reading you have is an impedance. Impedances are given in
| series form. If you shunt this with -j35, you should read 30.6 -j35
|
|
| If you want to tune out the reactance, put your -j35 capacitor in
| series with the load.
|

Wes, prior to my change of QTH from Massachusetts to New Hampshire I was
one of those guys using a center fed doublet with tuned 600 ohm feeders
and 40:1 VSWRs. I don't have any problem with non resonant antennas
and/or high VSWR! Just manage the losses at a level you are comfortable
with.

I also posted that I would not be concerned with a 40 +/- j35 load [VSWR
= 2.1:1]. If it causes the original poster problems then I stated 'trim'
the antenna.

Or, add/subtract feedline to eliminate the reactive component. Or, use a
transmission line stub to both eliminate the reactive component and
achieve a 1:1 VSWR. There are lots of methods to get to 1:1, but I
believe it is not necessary.

DD, W1MCE
+ + +

Wes Stewart wrote:

But in principle I agree the best place to do the
matching is at the antenna. But when I say this, all of the guys
using ladder line with 50:1 SWRs get all upset.