Mobile balun thoughts
 

I'm almost finished installing a bugcatcher antenna on my Vitara mini SUV.

I'm using a GLA 480 coil, 4 foot stainless bottom mast, and either 60
inches of AL top whip or a similar SS whip. I'll use a 12 to 16 inch cap
hat on top of the whip

Should I put a 1:1 balun on the setup or not?

- 73 de Mike KB3EIa -

Mobile balun thoughts
 

On Thu, 27 Sep 2007 14:42:30 -0400, Michael Coslo
wrote:

Should I put a 1:1 balun on the setup or not?

Hi Mike,

RF in the cab will keep you awake during long night drives. Just sit
on the mike enough that if you start to nod off, it fires up the rig
and the seat of your pants both.

Seriously, asking this one question only opens Pandora's box to all
the ways RF leakage and common mode can stop you dead in the road
(blame modern automotive technology). Your mini SUV is the major
radiator, the "antenna" is merely a tuned radial. There are some
elements of a Faraday cage here, but there are also elements of where
the cage fails, so does your control of RF in the cab, under the hood,
in the glove box, what-have-you.

73's
Richard Clark, KB7QHC

Mobile balun thoughts
 

Michael Coslo wrote:

Should I put a 1:1 balun on the setup or not?

- 73 de Mike KB3EIa -

Mike,

I'm sure there are a lot of opinions but here is what I do. Assuming
your mobile antenna is pretty well matched and you are running 100W then
the differential current in the coax will be about 1.4A. I have an
industrial clamp-on current probe that connects to a small battery
powered RF power meter (you can use something else) and I would measure
the common mode current on the shield of the coax. If the common mode
current was greater than 10% of the differential current in the coax
(0.14A) on any band of interest then I would use a good quality 1:1
balun. Why 10%? I have no real reason but I have done many mobile
installations and have nearly 30K mobile qso's and have no equipment
interactions or problems.

Obviously if the common mode current is minuscule then there no value in
having a balun. Also using a balun will do no harm and only cost you a
fraction of a dB. in loss.

73,
Larry, W0QE

Mobile balun thoughts
 

Richard Clark wrote in
:

On Thu, 27 Sep 2007 14:42:30 -0400, Michael Coslo
wrote:

Should I put a 1:1 balun on the setup or not?

Hi Mike,

RF in the cab will keep you awake during long night drives. Just sit
on the mike enough that if you start to nod off, it fires up the rig
and the seat of your pants both.

Would encourage me to shower more than once a day too...



Seriously, asking this one question only opens Pandora's box to all
the ways RF leakage and common mode can stop you dead in the road
(blame modern automotive technology). Your mini SUV is the major
radiator, the "antenna" is merely a tuned radial. There are some
elements of a Faraday cage here, but there are also elements of where
the cage fails, so does your control of RF in the cab, under the hood,
in the glove box, what-have-you.

I suppose that I could try with and without,and just find out.
There is at least one 2 meter Frequency that kicks me out of overdrive.
Fortunately it is on a repeater that I only use when out of town.

- 73 de Mike KB3EIA

Mobile balun thoughts
 

Larry Benko wrote in
:

Obviously if the common mode current is minuscule then there no value in
having a balun. Also using a balun will do no harm and only cost you a
fraction of a dB. in loss.


I'll give both ways a try and see what happens.

I forgot to mention that under Richard's mic sitting technique (in his
last post) that I would have to make sure to avoid eating at Taco Bell,
then accidentally leaving VOX on while driving.......

- 73 de Mike KB3EIA -

Mobile balun thoughts
 

On Thu, 27 Sep 2007 20:21:38 -0500, Mike Coslo
wrote:

There is at least one 2 meter Frequency that kicks me out of overdrive.

Hi Mike,

The cabin of a car or truck makes a fairly poor Faraday Shield to 2M.
Try as you might to correct things through a choke (conduction), there
may still be problems (radiation).

It is at this point that most vehicle owners discover all those parts
of their vehicle that are NOT grounded (or, rather, continuously
shielded). Hoods are usually floating with respect to the chassis and
make a great capacitor to the interior of the engine compartment.
Exhaust systems similarly float and are sometimes resonant in upper
HF. Doors hinge on plastic bushings... and the list continues.

73's
Richard Clark, KB7QHC

Mobile balun thoughts
 

Richard Clark wrote:
On Thu, 27 Sep 2007 20:21:38 -0500, Mike Coslo
wrote:

There is at least one 2 meter Frequency that kicks me out of overdrive.

Hi Mike,

The cabin of a car or truck makes a fairly poor Faraday Shield to 2M.
Try as you might to correct things through a choke (conduction), there
may still be problems (radiation).

It is at this point that most vehicle owners discover all those parts
of their vehicle that are NOT grounded (or, rather, continuously
shielded). Hoods are usually floating with respect to the chassis and
make a great capacitor to the interior of the engine compartment.
Exhaust systems similarly float and are sometimes resonant in upper
HF. Doors hinge on plastic bushings... and the list continues.


I have read about the need to use some copper braid to ground a lot of
the parts on the car. The exhaust system is certainly around the length
of the antenna mast in itself.

- 73 de Mike KB3EIA -

Mobile balun thoughts
 

Larry Benko wrote:
Michael Coslo wrote:

Should I put a 1:1 balun on the setup or not?

- 73 de Mike KB3EIa -

Mike,

I'm sure there are a lot of opinions but here is what I do. Assuming
your mobile antenna is pretty well matched and you are running 100W then
the differential current in the coax will be about 1.4A. I have an
industrial clamp-on current probe that connects to a small battery
powered RF power meter (you can use something else) and I would measure
the common mode current on the shield of the coax. If the common mode
current was greater than 10% of the differential current in the coax
(0.14A) on any band of interest then I would use a good quality 1:1
balun. Why 10%? I have no real reason but I have done many mobile
installations and have nearly 30K mobile qso's and have no equipment
interactions or problems.



HI Larry,

As an update, and not specifically about th ebalun issue, I tweaked my
system over the weekend.

Noise was an issue. When I first installed the antenna, it was on a
fairly quiet day, RF wise. The unmodified Vitara pumped a slew of noise
onto the radio. Without the engine running, I was seeing around S3-4
Noise. Turn the ignition on, and we're looking at S-9 worth of noise.
Obviously not too cool.

I installed ferrite snap ons onto various lines, such as the alternator,
the power lines to the rig, the power to the radiator fan. I then ran
wire braid from the hood to ground, and grounded all the doors the same way.

Yesterday afternoon, I tried a test drive of the system. Much much
quieter, although I hesitate to say exactly how much, since the band was
a lot noisier to begin with.

I still have to ground the exhaust system, and try a ferrite on the fuel
pump, which puts a little noise into the system. I still have to find
the fuel pump, though. I'm assuming it is on the tank - but I think a
call to Suzuki is in order.

40 meter operation was quite successful as far as the antenna working.
Good reports all around.

As also expected, there were a few in-vehicle issues.

One minor one is that the oil pressure light turned into a nice little
modulation indicator. This was whether the vehicle was running or not.
I'll probably put a ferrite on the power line to it in hopes of knocking
that out.

The other issue was that my voice would come out of the car stereo
speakers. A bit distorted but legible. I'm not sure if this was the
signal getting rectified somewhere or what. It would do this regardless
of if the stereo was turned on or not- thought oddly, it would start
only when the vehicle was turned on, and would go away gradually. At any
rate, I'l have to be sure to get that one fixed.

Finally, and this is not noise related, but I put a capacity hat on the
antenna as suggested by many. Being a homebrewer, I used what would best
be described as a "big freakin' pizza pan", a 16 inch pan mounted
upside down.

Turns out that I can adjust quite a bit lower in frequency than I
expected, and I will definitely have to shorten something up to run
higher than 20 meters.

- 73 de Mike KB3EIA -