antenna question
 

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?


Paul

A friend of mine let me borrow one of those MFJ antenna analizier

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?

The MFJ analyzer does not give the sign of the jX component of the impedance.
If the jX component is 35 ohms inductive reactance, then 35 ohms capacitive
reactance will cancel. The instruction manual says it's up to you to figure
the sign of jX.
In general, if you increase frequency and jX decreases, jX is capacitive.
Converse, inducutive.
73 Gary N4AST

"PDRUNEN" wrote in message
...
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?


Paul


unless the x is already capacitive, in which case it will get larger. that
thing doesn't tell the sign of the reactance only its magnitude.

As JG says: vary the frequency to determine the sign of X.

Increasing frequency and decreasing X means it is Capacitive Reactance.

Increasing frequency and increasing X means it is Inductive Reactance.

DD, W1MCE
+ + +

JGBOYLES wrote:

A friend of mine let me borrow one of those MFJ antenna analizier


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?


The MFJ analyzer does not give the sign of the jX component of the impedance.
If the jX component is 35 ohms inductive reactance, then 35 ohms capacitive
reactance will cancel. The instruction manual says it's up to you to figure
the sign of jX.
In general, if you increase frequency and jX decreases, jX is capacitive.
Converse, inducutive.
73 Gary N4AST

that thing doesn't tell the sign of the reactance only its magnitude.

David,
That thing! My most treasured piece of ham test gear. For the $180 I gave
for the 259B 4 years ago, it has given me the most enjoyment and info in my 40
years of this hobby. For the amount of money, it does a lot. We are amateurs,
right?


73 Gary N4AST

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.

"JGBOYLES" wrote in message
...
that thing doesn't tell the sign of the reactance only its magnitude.

David,
That thing! My most treasured piece of ham test gear. For the $180 I
gave
for the 259B 4 years ago, it has given me the most enjoyment and info in
my 40
years of this hobby. For the amount of money, it does a lot. We are
amateurs,
right?


73 Gary N4AST

yep, is a great tool. it helped me do a retune of my 80m 4-square one
handed that would have taken much longer with just swr bridges and a helper.
i don't know how i have lived this long without one... and i don't miss the
sign of the reactance, in most cases it either doesn't matter or you can
pretty much guess what it is.

JGBOYLES wrote:

that thing doesn't tell the sign of the reactance only its magnitude.


David,
That thing! My most treasured piece of ham test gear. For the $180 I gave
for the 259B 4 years ago, it has given me the most enjoyment and info in my 40
years of this hobby. For the amount of money, it does a lot. We are amateurs,
right?


73 Gary N4AST

'that thing' is the best piece of non professional laboratory grade RF
equipment I own.

Take a little time to study what it will do for you. You'll be surprised!!

Dave, W1MCE
Manager, Textron Antenna Range, Textron Defense Systems, retired

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.
|