Re: Re: B40 rf interference problem

[mborgelt]

John,

The advice about rf floating around your cockpit is all good.

With a good radio/co-ax/antenna the B40 with DAD should not be having
any transmit troubles.

We had one B40 in an ASW19 in Japan that had a similar problem which
was traced to improper termination of the BNC connector on the coax
where it plugged in to the radio. Fixing the coax problem made the rf
problem go away.

When putting a BNC connector on to a piece of coax refer to the
manufacturers' instructions. It isn't at all obvious how it should be
done properly.

Mike Borgelt


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[John]

SWR measured 1.25 and ~38 ohms at 123 Mhz. I connected the instrument
directly to the antenna using the bnc that gets connected to the radio.
Does this also prove the bnc conenctor is good or do I need to
disassemble it?

Thanks

[Eric Greenwell]

John wrote:
> SWR measured 1.25 and ~38 ohms at 123 Mhz. I connected the instrument
> directly to the antenna using the bnc that gets connected to the radio.

An SWR of 1.25 is nearly perfect, but where did the 38 ohms number come
from? I've not seen an SWR meter that shows the antenna impedance, which
is normally 50 ohms at 123 mhz. Was the ohms measurement with an ohmmeter?

> Does this also prove the bnc conenctor is good or do I need to
> disassemble it?

Let's hear where the 38 ohm number came from, first!

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Eric Greenwell
Washington State
USA

[John Giddy]

On 12 Mar 2005 09:13:43 -0800, John wrote:

> SWR measured 1.25 and ~38 ohms at 123 Mhz. I connected the instrument
> directly to the antenna using the bnc that gets connected to the radio.
> Does this also prove the bnc conenctor is good or do I need to
> disassemble it?
>
> Thanks

Seems a pretty good reading. Yes, it will include the connector.
What type of radio is it ? From your symptoms there does seem to be a
strong RF field near the vario. Maybe the radio itself is leaking RF ?
An alternative source of the RF could be the power wiring to the
radio. Does this run close to the wires for the averager ?
If so, try separating the two sets of wires by a few inches, and see
if that makes a difference.
Cheers, John G.

[John Giddy]

On Sat, 12 Mar 2005 12:50:43 -0800, Eric Greenwell wrote:

> John wrote:
>> SWR measured 1.25 and ~38 ohms at 123 Mhz. I connected the instrument
>> directly to the antenna using the bnc that gets connected to the radio.
>
> An SWR of 1.25 is nearly perfect, but where did the 38 ohms number come
> from? I've not seen an SWR meter that shows the antenna impedance, which
> is normally 50 ohms at 123 mhz. Was the ohms measurement with an ohmmeter?
>
>> Does this also prove the bnc conenctor is good or do I need to
>> disassemble it?
>
> Let's hear where the 38 ohm number came from, first!

Eric,
38 ohms is probably what the SWR meter showed. It is about the
impedance which would give a 1.25 SWR in a 50 ohm system.
I haven't done the math, but from a couple of charts, that's what I
would assume.
Cheers, John G.

[Marc Ramsey]

John Giddy wrote:
> An alternative source of the RF could be the power wiring to the
> radio. Does this run close to the wires for the averager ?

Another possible source of RF leaks is the wiring to the push-to-talk
switch.

Marc

[John]

The SWR analyzer I used can also measure the antenna's impedance.

The radio is a Becker AR3201. The power wiring for the averager is not
close to the radio wiring because the radio is the bottom of the
intrument stack and the B40 is at the very top (> 12-14" away). I tried
to trace the antenna's coax back along the inside of the fuse and
determined that the cable must end somewhere between the control stick
access and the access panel in the deck behind the pilot's head. It's a
DG-300 and there is no seat pan that can be removed (great design).
Anyway, I suspect that means the antenna is located under the permanent
seat pan. Where would the antenna normally be located and is it's
distance to the panel a potential cause of my problems?

[Marc Ramsey]

John wrote:
> Anyway, I suspect that means the antenna is located under the permanent
> seat pan. Where would the antenna normally be located and is it's
> distance to the panel a potential cause of my problems?

The antenna in DG-300s (and most glass gliders) is located in the
vertical fin. The coax is running through a conduit under the seat pan,
then out through the tail boom...

[John]

Marc,

You're absolutelty correct. I missed the coax the first time I stuck my
head down the boom. It's behind a bulkhead and glass'd into the side.

Thanks

[Malcolm Austin]

Hi,
to answer the final question; "Where would the antenna normally
be
located and is it's > distance to the panel a potential cause of my
problems?"

If the coax is of good quality, it should not radiate RF. If its of dubious
quality it will radiate and then it can become important for the length
to be multiples of a 1/4 wave (or 1.902 feet per 1/4 wave at 123 mHz).

One way to work out the efficiency of the coax it to put a dummy
load at the antenna end (provided you can get there) and work out
the loss of power from the radio end to the far end. It can be a real
surprise how much you lose (on one test I ran, it was a 75% loss over
a 60 foot length!)

Malcolm...

"John" > wrote in message
ups.com...
> The SWR analyzer I used can also measure the antenna's impedance.
>
> The radio is a Becker AR3201. The power wiring for the averager is not
> close to the radio wiring because the radio is the bottom of the
> intrument stack and the B40 is at the very top (> 12-14" away). I tried
> to trace the antenna's coax back along the inside of the fuse and
> determined that the cable must end somewhere between the control stick
> access and the access panel in the deck behind the pilot's head. It's a
> DG-300 and there is no seat pan that can be removed (great design).
> Anyway, I suspect that means the antenna is located under the permanent
> seat pan. Where would the antenna normally be located and is it's
> distance to the panel a potential cause of my problems?
>