Hi de KJ4UO,

I have a MFJ-269 and I measured a value of 65 and j16 for my 6-meter
dipole, the dipole is a little long so I assume that the j16 is
inductive and I need to add a cap of equal reactive value.
Also, The measurement was taken directly at the antenna feedpoint
without any coax.

Once I back calculate the cap value ,should I place this cap across the
antenna input at the feed point or in series with the signal line of
the feed point to cancel out the j16 and leave me with 65 ohm
resistive?


tnx de KJ4UO

wrote:
Hi de KJ4UO,

I have a MFJ-269 and I measured a value of 65 and j16 for my 6-meter
dipole, the dipole is a little long so I assume that the j16 is
inductive and I need to add a cap of equal reactive value.
Also, The measurement was taken directly at the antenna feedpoint
without any coax.

Once I back calculate the cap value ,should I place this cap across the
antenna input at the feed point or in series with the signal line of
the feed point to cancel out the j16 and leave me with 65 ohm
resistive?


tnx de KJ4UO

Adding a cap with 16 ohms reactance in series will bring your impedance
to 65 ohms resistive. Adding a 280 ohm reactance cap in parallel will
cancel the inductance but the resistive component will now be about 69 ohms.

Why not just trim the dipole a bit?

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

wrote:
I have a MFJ-269 and I measured a value of 65 and j16 for my 6-meter
dipole, the dipole is a little long so I assume that the j16 is
inductive and I need to add a cap of equal reactive value.
Also, The measurement was taken directly at the antenna feedpoint
without any coax.

How did you keep your body from affecting the measurement?
Why do you think an SWR of 1.5:1 is enough to worry about?

Once I back calculate the cap value ,should I place this cap across the
antenna input at the feed point or in series with the signal line of
the feed point to cancel out the j16 and leave me with 65 ohm
resistive?

The impedance displayed by the MFJ-269 is a series impedance.
If the feedpoint impedance is indeed 65+j16, two caps of -j8,
one to each side of the dipole on the antenna side of a 1:1
choke-balun will tend to keep the currents balanced.

But here's another approach for your consideration. A little
less than 1/2WL of 300 ohm or 450 ohm line will bring the antenna
to resonance, assuming a 65+j16 feedpoint impedance. That's about
8.5 feet on 6m. A series section of ladder-line is usually easier
to install than discrete components. And if you did make an error
in your measurement, trimming the ladder-line to resonance is a
pretty easy task.
--
73, Cecil http://www.qsl.net/w5dxp

wrote:
Hi de KJ4UO,

I have a MFJ-269 and I measured a value of 65 and j16 for my 6-meter
dipole, the dipole is a little long so I assume that the j16 is
inductive and I need to add a cap of equal reactive value.
Also, The measurement was taken directly at the antenna feedpoint
without any coax.

Once I back calculate the cap value ,should I place this cap across the
antenna input at the feed point or in series with the signal line of
the feed point to cancel out the j16 and leave me with 65 ohm
resistive?

There's no point in adding a capacitor at all. Assuming you're using a
50 ohm feedline, it would only change the SWR from about 1.5 to 1.3. I
can't see where that would be worth the trouble.

But you haven't measured the dipole impedance. One terminal of the
MFJ-269 is connected to one side of the dipole. The other terminal is
connected to the dipole in parallel with the outside of the MFJ-269
itself and, if you were holding it, your body also. So you're measuring
the impedance of an antenna having a wire for one leg, and a combination
of a wire, box, and body for the other.

If you really had measured the dipole impedance and if the reactance was
enough to bother with, the answer to your question would be to put it in
series.

Roy Lewallen, W7EL

Roy quite reasonably wrote, "If you really had measured the dipole
impedance and if the reactance was enough to bother with, the answer to
your question would be to put it in
series."

You can instead put capacitance in parallel with the nominal 65+j16 (if
that's what it is) to make it non-reactive, though doing so will raise
the resistive impedance, and thus not improve the SWR rel. to 50 ohms
as much as series capacitance (a pair of balanced caps...). But if you
increase the shunt capacitance beyond what's needed to cancel the
reactance, and then add series inductance, you can get to a "perfect"
match to 50 ohms. (There are, in general, many ways to match one
impedance to another, by using various combinations of inductance,
capacitance, transmission line stubs and transmission line series
sections.)

But first beware of the pitfalls Roy mentioned, and understand if you
really need additional matching.

Cheers,
Tom

Try suspending the measuring instrument in mid-air to remove your body
capacitance and see if it makes any difference to the measured
impedance.
----
Reg

Reg Edwards wrote:
Try suspending the measuring instrument in mid-air to remove your body
capacitance and see if it makes any difference to the measured
impedance.

Suspending the analyzer might be adequate for 6 meters, which is the
antenna under discussion. But I should mention for the benefit of the
general audience that it isn't adequate for 2 meters. Not too long ago I
heard from an EZNEC user who found a substantial difference between
modeled and measured results from a simple antenna. On my suggestion, he
added a simple wire frame representation of the MFJ 269 to his model,
and it made the model results much closer to his measurement. So in that
case the antenna analyzer itself was enough to substantially skew the
measurement.

Roy Lewallen, W7EL

The Autec Analyser is a better instrument than the MFJ for
suspended-in-mid-air measurements. It is much smaller and is lighter
in weight and has a smaller capacitance.

The Autec case has a self-capacitance of the order of 3 pF
corresponding to a reactance of 1768 ohms at 30 MHz which can be
ignored when measuring 50 ohms with zero lead length.

The Autec's highest frequency is 35 MHz.

The MFJ's highest frequency is at VHF. With a self-capacitance of 7
pF at VHF substantial errors can occur.

To estimate capacitance, the DC capacitance of a sphere is -

pF = 55.55 * Diameter in metres.
----
Reg.

Reg wrote, "Try suspending the measuring instrument in mid-air to
remove your body capacitance and see if it makes any difference to the
measured impedance."

Even better (perhaps), set up the antenna as you plan to use it,
including balun and feedline. Make your measurements at the "other"
(non-antenna) end of the feedline. Back out the effects of that length
of feedline, and you have the antenna feedpoint impedance in that
particular environment. If the line is reasonably long so you and your
measurement device are at least a couple wavelengths from the antenna,
and reasonably well decoupled from the antenna so that there isn't a
lot of "antenna current" on the feedline, your measurements should be
essentially independent of your presence and the presence of the
analyzer.

Cheers,
Tom

Hi again all,

the dipole was suspensed on a pole and the MFJ-269 was connected to it,
no body effects here. After adjusting frequency then I move away from
the setup.

I do understand that 65 + j16 is not really a bad match, if I did add a
capacitive reactance, then would I have to place two caps, each of
equal value in series with coax?

If I wanted to do a shunt capacitor, would I find the admittance of the
above and then determine a shunt from the resulting reactance?

de KJ4UO