Can someone please provide guidance for this issue:

a snippet from the SGC website:
"One of the great misunderstandings in checking grounds is
that just because you measure continuity with a VOM, that you will have
a good RF ground. You MAY have a good RF ground, but a DC measurement
doesn't prove it. Remember, if you measure the resistance across an
inductor, it will show zero DC resistance to ground even though it's
impedance at HF frequencies may be quite high. Continuity is a good
check, but it does not certify your RF ground system."

I would like to understand how/what to definitively measure before and
after I make improvements to my mobile HF system (additional
bonding/grounding). What metric will provide clear indication that I
have indeed made an improvement. I do have an MFJ-259 available. I
have been exclusively looking at the SWR for indication but I am
starting to learn that lowering the SWR is not necessarily an
improvement. Is it to simply to measure the mobile vertical feedpoint
impedance before and after ?

73, dennis, k1drw

k1drw wrote:
Can someone please provide guidance for this issue:

a snippet from the SGC website:
"One of the great misunderstandings in checking grounds is
that just because you measure continuity with a VOM, that you will have
a good RF ground. You MAY have a good RF ground, but a DC measurement
doesn't prove it. Remember, if you measure the resistance across an
inductor, it will show zero DC resistance to ground even though it's
impedance at HF frequencies may be quite high. Continuity is a good
check, but it does not certify your RF ground system."

I would like to understand how/what to definitively measure before and
after I make improvements to my mobile HF system (additional
bonding/grounding). What metric will provide clear indication that I
have indeed made an improvement. I do have an MFJ-259 available. I
have been exclusively looking at the SWR for indication but I am
starting to learn that lowering the SWR is not necessarily an
improvement. Is it to simply to measure the mobile vertical feedpoint
impedance before and after ?

73, dennis, k1drw


Hi Dennis,

Have you thought about field strength measurements?

Cecil, where are you?

Chuck

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Measure the feedpoint impedance with your MFJ. The resistance part of
the measurement is the total of the radiation and loss resistance. A
lower value of R means you've reduced the loss resistance by the amount
of the decrease.

Roy Lewallen, W7EL

k1drw wrote:
Can someone please provide guidance for this issue:

a snippet from the SGC website:
"One of the great misunderstandings in checking grounds is
that just because you measure continuity with a VOM, that you will have
a good RF ground. You MAY have a good RF ground, but a DC measurement
doesn't prove it. Remember, if you measure the resistance across an
inductor, it will show zero DC resistance to ground even though it's
impedance at HF frequencies may be quite high. Continuity is a good
check, but it does not certify your RF ground system."

I would like to understand how/what to definitively measure before and
after I make improvements to my mobile HF system (additional
bonding/grounding). What metric will provide clear indication that I
have indeed made an improvement. I do have an MFJ-259 available. I
have been exclusively looking at the SWR for indication but I am
starting to learn that lowering the SWR is not necessarily an
improvement. Is it to simply to measure the mobile vertical feedpoint
impedance before and after ?

73, dennis, k1drw

Roy knows whereof he speaks... Just for fun let me stir the pot here
and introduce a different way of looking at the mobile antenna
system...

The mobile antenna can be viewed as an off center fed dipole (OCF)...
The mast and top hat form one side of the dipole and the vehicle (the
so called ground) the other side... The problems I see - besides the
low radiation resistance of the electrically short mast and "ground" -
is that the L/D ratio of the two parts of the antenna are vastly
different...
The mast portion has an LD ratio of ~20 picked out of thin air, I am
sure someone will correct me
The "ground" has a ratio of less than 1.0
This massive offset in L/D between the two parts of the antenna system
results in massive skewing of the capacitative coupling to actual
ground on each half of the antenna...

What I propose is to not connect the RF of the antenna system directly
to the vehicle chassis... Use an insulated mast as usual, but do not
connect the coax braid to the vehicle... Instead, use the braid of RG8
as a radial/counterpoise to the mast... Run the coax underneath the
vehicle and wind it around the perimeter leaving it a shorter than an
electrical quarter wave... Leave it insulated from the vehicle... Seal
the ends against water and salt intrusion... Use a coil at the feed
point to establish resonance on this counterpoise... Compare the field
strength to the conventional method...

denny / k8do

k1drw wrote:
Can someone please provide guidance for this issue:

a snippet from the SGC website:
"One of the great misunderstandings in checking grounds is
that just because you measure continuity with a VOM, that you will have
a good RF ground. You MAY have a good RF ground, but a DC measurement
doesn't prove it. Remember, if you measure the resistance across an
inductor, it will show zero DC resistance to ground even though it's
impedance at HF frequencies may be quite high. Continuity is a good
check, but it does not certify your RF ground system."

I would like to understand how/what to definitively measure before and
after I make improvements to my mobile HF system (additional
bonding/grounding). What metric will provide clear indication that I
have indeed made an improvement. I do have an MFJ-259 available. I
have been exclusively looking at the SWR for indication but I am
starting to learn that lowering the SWR is not necessarily an
improvement. Is it to simply to measure the mobile vertical feedpoint
impedance before and after ?

73, dennis, k1drw


Dennis, assuming you've already done at least some "bonding and
grounding," further measures may have a fairly small incremental effect
on loss resistance and efficiency. The MFJ-259 has a resolution of one
ohm and the uncertainty of that LSD is probably more than an ohm. You
may not be able to reliably detect small improvements using the MFJ.

These issues may be less troublesome if you use field strength
measurements to detect changes.

Plan carefully, because once you make a change in the system, it is a
pain to have to un-do it in order to apply a different measurement
technique. Will you share your results?


73,

Chuck

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On 29 Dec 2006 04:52:47 -0800, "Denny" wrote:

What I propose is to not connect the RF of the antenna system directly
to the vehicle chassis... Use an insulated mast as usual, but do not
connect the coax braid to the vehicle... Instead, use the braid of RG8
as a radial/counterpoise to the mast...

Hi Denny,

Sounds like what Wall Street calls "taking a poison pill."

Run the coax underneath the
vehicle and wind it around the perimeter leaving it a shorter than an
electrical quarter wave... Leave it insulated from the vehicle... Seal
the ends against water and salt intrusion... Use a coil at the feed
point to establish resonance on this counterpoise...

Another poison pill.

Compare the field
strength to the conventional method...

If it can deflect the needle. You might have to duct tape the FSM to
this novel radiator to do that.

Care to guess what the circulating currents are going to be in your
insulated mast antenna?

73's
Richard Clark, KB7QHC

Roy, Denny, Chuck:
Thanks guys for the information. I feel I do understand now, how
to measure (using MFJ "R"/"X" and Field strength) to determine if any
improvements are accomplished. Yes, after I finish I will post a
follow up.

73, Dennis, k1drw

On Fri, 29 Dec 2006 09:04:01 -0500, chuck wrote:

The MFJ-259 has a resolution of one
ohm and the uncertainty of that LSD is probably more than an ohm. You
may not be able to reliably detect small improvements using the MFJ.

Hi Chuck,

This is rather focused at the wrong end of the scale. The antenna's
radiation resistance is going to be lost at the 1 Ohm resolution of
the scale. A 12 foot radiator in the 80M band presents us with about
5 Ohms (a 10:1 mismatch) resistance with so much reactance that it
would wrap the needle around the peg at the wrong end of the SWR
scale.

However, the "lucky" ones may in fact see a SWR that is closer to 2:1,
only to be satisfied that 20 of those Ohms comes from poor
connections, coax, heated coils, and chassis loss. We can all agree
that the MFJ is perfectly capable of measuring 20 Ohms without issues
of error (yes, there will be error, but it won't be focused in the
last digit). You will also be able to see that 20 Ohms diminish
TOWARDS the 5 Ohm Rr when you struggle to IMPROVE the situation. Even
if that is everyone's goal, it is rarely achieved; and certainly not
because accuracy foiled them.

73's
Richard Clark, KB7QHC

Richard Clark wrote:
On Fri, 29 Dec 2006 09:04:01 -0500, chuck wrote:

The MFJ-259 has a resolution of one
ohm and the uncertainty of that LSD is probably more than an ohm. You
may not be able to reliably detect small improvements using the MFJ.

Hi Chuck,

This is rather focused at the wrong end of the scale. The antenna's
radiation resistance is going to be lost at the 1 Ohm resolution of
the scale. A 12 foot radiator in the 80M band presents us with about
5 Ohms (a 10:1 mismatch) resistance with so much reactance that it
would wrap the needle around the peg at the wrong end of the SWR
scale.

However, the "lucky" ones may in fact see a SWR that is closer to 2:1,
only to be satisfied that 20 of those Ohms comes from poor
connections, coax, heated coils, and chassis loss. We can all agree
that the MFJ is perfectly capable of measuring 20 Ohms without issues
of error (yes, there will be error, but it won't be focused in the
last digit).

Where will the error be focused?

You will also be able to see that 20 Ohms diminish
TOWARDS the 5 Ohm Rr when you struggle to IMPROVE the situation. Even
if that is everyone's goal, it is rarely achieved; and certainly not
because accuracy foiled them.

73's
Richard Clark, KB7QHC

Hi Richard,

It would surprise me to see a 15 ohm reduction due to incremental
grounding and bonding. I would expect changes in R to be in the range of
a few ohms: right up against the uncertainty of the instrument.

But if he can bond a fender to the hood and see a 15 ohm reduction in
loss resistance, I admit to raising a non-issue. ;-) You and I are
simply expecting different performance results from the incremental
grounding and bonding.

Absolute accuracy is unimportant in this application.

73,
Chuck

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On Fri, 29 Dec 2006 13:37:43 -0500, chuck wrote:

Absolute accuracy is unimportant in this application.

Hi Chuck,

Quite so, hence the last digit is merely resolution, and the
resolution of 20 Ohms of loss against an Rr of 5 Ohms is easily
resolvable, and its reduction is easily seen. Further, automatically
assigning an error of 1 to the last digit presumes the underlying
measurement hasn't been rounded up from a smaller range of error.
Given the product is MFJ, it is hard to credit it with that kind of
craftsmanship. However, even with a 1 digit error (which has been
presumed to be quantization error), it will always be inherent in any
Relative reading. If 20 Ohms reading is actually 19 to 21 Ohms of
loss (irrespective of absolute accuracy), then driving out 1 Ohm of
loss will actually be 18 to 20 Ohms exhibited as 19 Ohms. The only
problem here is that you may have to drive out slightly more than 1
Ohm to kick the reading down to 19.

If you cannot drive out more than 1 Ohm of loss out of 20, then
there's no point in trying, is there?

Dennis,

If you've followed the discussion thus far, you can well expect that
the SWR will climb as you drive out loss. This means you need to
anticipate matching to something like a 5 Ohm load (in all likelihood,
as described here, it will be higher). Drive out the reactance and
use a 9:1 UnUn. Beware that such an UnUn will require 16 Ohm
transmission line (if it was all that easy, everyone would have
stopped posting similar problems like this long ago).

73's
Richard Clark, KB7QHC