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.

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?

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.

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


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.

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.