I am using a multi-switch to change radios and antennas. One of those MFJ
1700Bs. I get different SWR readings depending upon where I place the
bridge in the tangle of wires that go into and out of the switch. If I
take the switch out of the equation I get very low SWR. With it in I get
2.5:1.

All of the coax has a common ground so the OCF windom also shares a common
ground with the coax going up to the beam and all the radios. All grounds
are starred at the back of the antenna tuner and run to earth.
I am wondering - does the common connection mean the coax shield to the beam
is becoming a part of the OCF windom antenna and vice-versa?

Would it therefore be adviseable to put current chokes on all the ins and
outs of the switch? Other comments?

k4ia
Buck
Fredericksburg, VA

On Fri, 02 Mar 2007 15:06:40 GMT, "Teri Buck"
wrote:

I am using a multi-switch to change radios and antennas. One of those MFJ
1700Bs. I get different SWR readings depending upon where I place the
bridge in the tangle of wires that go into and out of the switch. If I
take the switch out of the equation I get very low SWR. With it in I get
2.5:1.

All of the coax has a common ground so the OCF windom also shares a common
ground with the coax going up to the beam and all the radios. All grounds
are starred at the back of the antenna tuner and run to earth.
I am wondering - does the common connection mean the coax shield to the beam
is becoming a part of the OCF windom antenna and vice-versa?

Would it therefore be adviseable to put current chokes on all the ins and
outs of the switch? Other comments?

k4ia
Buck


Based upon your description you can bet the farm you've got common
mode problems. Off center fed dipoles (that's what you have - its not
a windom) are notorious for that problem.

You'll need to install some good common mode chokes (high choking
impedance). For an OFCD you'll need two or more on the coax feed
line. I would start with one at the antenna feed point and another
where the coax enters the shack.

Danny, K6MHE

On Fri, 02 Mar 2007 07:25:39 -0800, Danny Richardson
wrote:

Based upon your description you can bet the farm you've got common
mode problems.

Amen

On 2 mar, 16:06, "Teri Buck" wrote:
I am using a multi-switch to change radios and antennas. One of those MFJ
1700Bs. I get different SWR readings depending upon where I place the
bridge in the tangle of wires that go into and out of the switch. If I
take the switch out of the equation I get very low SWR. With it in I get
2.5:1.

Hello Teri,

It is very likely a common mode problem (as Danny and Richard also
mentioned).

Sometimes the SWR meter is the problem. Check your instrument by
connecting a known mismatched with various length of cable (for
example 75 Ohms resistor in combination with 2 1/8 wave 50 Ohms
cables). This resistor should give a 1.5 reading on the instrument,
irrespective of cable length between resistor and instrument (provided
that cable losses can be neglected).

Other problem (unlikely) can be significant harmonics in the output of
your Final Amplifier.

Best regards,
Wim
PA3DJS

"Teri Buck" wrote in news:44XFh.4$mI6.3@trndny08:

I am using a multi-switch to change radios and antennas. One of those
MFJ 1700Bs. I get different SWR readings depending upon where I place
the bridge in the tangle of wires that go into and out of the switch.

Teri, some thoughts:

For practical transmission lines and antennas, the VSWR decreases
smoothly from load to source, and the rate of decrease depends on the
transmission line (one way) loss and is predictable.

For example if the VSWR is 1.5 at a certain point, then at another point
closer to the source and where the one way cable loss is 1dB to the first
point, the VSWR will be 1.4.

So, if you are truly making VSWR readings without changing the line
length, you should expect the readings to decrease towards the generator,
but the decrease for practical lines will be quite small. If you observe
otherwise (higher or lower) something else is happening.

A well designed switch should not change the VSWR much on the through
leg, so measurements either side of it should be similar.

If you are changing line length as well as moving the VSWR meter (just to
complicate issues), a likely contiribution is that the outside of the
outer conductor is influencing the load seen at the load end of the coax,
ie the cable is carrying current on its outer (which is often
undesirable).

If I take the switch out of the equation I get very low SWR. With it
in I get 2.5:1.

Is this on only one antenna, or are you generalising and oversimplifying
things?

In taking the switch out, have you disconnected the cable outer
conductors whereas they would have all been bonded at the switch?

What does the switch do with the non-selected antennas, does it ground
them? Grounding the end of the feedline to the unused, but mutually
coupled antenna may inflence the load presented by the selected antenna.

Have you checked all the components in your "tangle of wires". Are there
faulty cables, or loose connectors (especially if you are using UHF
connectors).

The answers to these questions might throw light on what is happening and
whether choking will fix it.


All of the coax has a common ground so the OCF windom also shares a

Well, you know that the "OCF Windom" (aren't they all off centre fed)
depends on a radiating feedline. Perhaps you mean one of the newer
designs with coax vertical that are designed to have common mode current
on the vertical part, then you try to restrict that current at a point
where you decide the coax is to be feedline and not radiator, which is
easier said than done.

....

Owen