On Sun, 3 Sep 2006 10:32:41 -0500, "Bob D."
wrote:

I'm about to put up a V2000 6m/2m/70cm antenna. I've used 9913 in the past
for 2m, but it's awfully stiff and expensive. I'm thinking of going with RG8
or a variant of RG8. Are they all the same? I want to minimize loss of
course. Recommendations?

BTW, what does the RG number signify anyway? The size? The construction?

The Wireman's Wirebook IV gives a pretty good history of coax: Going
back to the 1940s, R = Radio Frequency; G = Government; 8 is a
sequential number assigned for government approval, thus RG8/U with U
= Universal Specification.

That said, all of these terminologies are fairly obsolete to the
modern manufacture of coax, according to the Wireman.

bob
k5qwg

Yeh, BUT, some coax has a smaller amount of copper
in the shield, and is therefore lossier at VHF/UHF
freqs. tho probably ok at HF. Also, look at the
RG number on the outer shieth of the coax. IF
it says " RG- 8 TYPE" instead of RG8, avoid it
like the plague , if you want to use it at VHF-UHF
, and "RG-8X" is another horse of a different
color, too, at vhf. Also, remember, that if you
are going to use for impedence transformation,
that the velocity factor of Poly center insulator
is .66 X the desired wavelength, while , Foam
dielectric, is .8 X the desired wavelength.
(No insulation, gives a velocity factor of 1.0).
In short: you get what you pay for (no free lunch)
Jim NN7K


G. Doughty wrote:
I just picked up 200 ft of rg8 at radio shack for 40 bucks. 10 bucks for 50
foot run. Anyways, there was a website I came across recently that showed
the swr rating of different size rg cables based on band. I searched for
rg8 and the site popped up. It was complete with charts, etc. Very nice.
It only takes a few minutes to find if you don't mind trudging through the
sites.

Greg
ki4bbl

Roy Lewallen wrote:
G. Doughty wrote:
I just picked up 200 ft of rg8 at radio shack for 40 bucks. 10 bucks
for 50 foot run. Anyways, there was a website I came across recently
that showed the swr rating of different size rg cables based on band.

What's an SWR rating? The cable with the most loss will have the best
SWR when feeding a mismatched load.

Roy Lewallen, W7EL

Not to mention that, at VHF (and other freqs, if
you are so unfortunate, as to have cut the coax
length just right, you can show a GREAT SWR, and
still be in for problems (this like pruneing
a coax for the "Best Match")! A way to Double
Check for this, is to use a 1/4 wave (electrical)
length of coax , OF the SAME impedance, in series
with your feed line to the antenna. IF the SWR
stays the same, Should have a match, BUT IF the
SWR goes up, better check for match problems!
As info, Jim NN7K

On Mon, 04 Sep 2006 22:15:34 GMT, Jim - NN7K wrote:


a coax for the "Best Match")! A way to Double
Check for this, is to use a 1/4 wave (electrical)
length of coax , OF the SAME impedance, in series
with your feed line to the antenna. IF the SWR
stays the same, Should have a match, BUT IF the
SWR goes up, better check for match problems!
As info, Jim NN7K

Jim, can you explain what is going on here, why what happens happens.

It has more to do with what is happening on the outside of the coax
than on the inside of the coax.

Owen
--

Quite easily. Impedence matches repeat, every 1/2
wavelength. This is the basis for those old "CB"
shops , pruning the coax for the best match
(rather then the antenna)! At ODD multiples of a
1/4 wavelength. the coax, becomes a MATCHING
Transformer (like stacking 2- 50 ohm antennas,
you use 1/4 wavelength sections of 75 ohm coax)
In this case, we are matching 2 -50 ohm loads to
a 25 ohm source)-- Coax has a characteristic
impedence that is constant along its length.
as long as the Source Impedence = the Load
Impedence. IF the source is DIFFERENT from the
load impedence, or the Coax has a different
impedence, from the source and/or load, however
the source/coax/and load will show 1:1 SWR,
AT the 1/2 wave points of the line, repeating
EVERY 1/2 wavelength! However, as stated, ODD
1/4 wavelengths become a impedence XFMR.
(the formula being : X Zo (1/4 wave) = the
SQUARE ROOT of R1 x R2! Was well documented in
an a old VHF'er article from the '60's!
Ill post it (in text format, when I find it-- Jim


Owen Duffy wrote:
On Mon, 04 Sep 2006 22:15:34 GMT, Jim - NN7K wrote:


a coax for the "Best Match")! A way to Double
Check for this, is to use a 1/4 wave (electrical)
length of coax , OF the SAME impedance, in series
with your feed line to the antenna. IF the SWR
stays the same, Should have a match, BUT IF the
SWR goes up, better check for match problems!
As info, Jim NN7K

Jim, can you explain what is going on here, why what happens happens.

It has more to do with what is happening on the outside of the coax
than on the inside of the coax.

Owen
--

On Mon, 04 Sep 2006 23:31:00 GMT, Jim - NN7K wrote:

Quite easily. Impedence matches repeat, every 1/2
wavelength. This is the basis for those old "CB"
shops , pruning the coax for the best match
(rather then the antenna)! At ODD multiples of a
1/4 wavelength. the coax, becomes a MATCHING
Transformer (like stacking 2- 50 ohm antennas,
you use 1/4 wavelength sections of 75 ohm coax)
In this case, we are matching 2 -50 ohm loads to
a 25 ohm source)-- Coax has a characteristic
impedence that is constant along its length.
as long as the Source Impedence = the Load
Impedence. IF the source is DIFFERENT from the
load impedence, or the Coax has a different
impedence, from the source and/or load, however
the source/coax/and load will show 1:1 SWR,
AT the 1/2 wave points of the line, repeating

If the SWR on a line is 1:1 at any point, it is 1:1 at ALL points on
the line.

If the SWR on a low loss line is close to 1:1 at any point, it is
approximately the same at ALL points on the line.

On practical lines at HF and above with practical loads, the SWR
decreases smoothly from the load to the source because of line loss,
the lower the loss, the lower the rate at which SWR falls as you
approach the source.

If lengthening the line increases the SWR at the tx, or decreases it
by more than accounted for by the extra line loss, then something else
is happening.

Owen


EVERY 1/2 wavelength! However, as stated, ODD
1/4 wavelengths become a impedence XFMR.
(the formula being : X Zo (1/4 wave) = the
SQUARE ROOT of R1 x R2! Was well documented in
an a old VHF'er article from the '60's!
Ill post it (in text format, when I find it-- Jim


Owen Duffy wrote:
On Mon, 04 Sep 2006 22:15:34 GMT, Jim - NN7K wrote:


a coax for the "Best Match")! A way to Double
Check for this, is to use a 1/4 wave (electrical)
length of coax , OF the SAME impedance, in series
with your feed line to the antenna. IF the SWR
stays the same, Should have a match, BUT IF the
SWR goes up, better check for match problems!
As info, Jim NN7K

Jim, can you explain what is going on here, why what happens happens.

It has more to do with what is happening on the outside of the coax
than on the inside of the coax.

Owen
--
--

The answer to the subject question is: NO!

Bob D. wrote:

I'm about to put up a V2000 6m/2m/70cm antenna. I've used 9913 in the past
for 2m, but it's awfully stiff and expensive. I'm thinking of going with RG8
or a variant of RG8. Are they all the same? I want to minimize loss of
course. Recommendations?

BTW, what does the RG number signify anyway? The size? The construction?

Owen
Keep in mind that I'am (primarily) VHF/UHF op.
You might find this interesting (- the drawings)!
This from the VHF'er, Nov, 1964-- Jim
ACID TEST FOR A FLAT TRANSMISSION LINE Loren Parks
K7AAD (BTW, Loren runs Parks Electronics, in
Portland, Or. and has given up ham radio long ago!)


The mark of a beginner is blind faith in meters.
Most hams get over believing an "S" meter after a
few years, but very few get to the point of
doubting that SWR or power meter. You ought to try
hooking half a dozen SWR bridges and power meters
in series with your transmission line--all at the
same time. If the line is not pro*perly terminated
you'll probably get as many different readings as
you have meters.
An SWR bridge or wattmeter can show zero reflected
"power" or nearly so on a badly mis-matched
transmission line. This is why the beginners
"prune" the transmission line to " get the SWR
down." Pruning can help you load into a badly
mismatched line but the seat of the trouble is in
the termination--the antenna.
The proof of a properly terminated transmission
line is that changing its length has no effect on
transmitter loading or on an indicator in the line
ahead of the length that is changed. If your
bridge now shows little or no reflected power or
voltage, it may be that it is inserted at a null.
Therefore you have to move the null, if any, by
chang*ing the distance from the meter to the load
(not the transmitter.) Inserting 1/4 wave*length
(electrically) of coax will give a maximum change
in meter reading.
Insert lengths as shown in the table below. Make
some allowance for the additional length of
fittings or adapters at 432 Mc. Dimensions are not
very critical.
Added length of Original length of
transmission line transmission line
Transmitter
SWR Bridge or Power Meter
Changing the length of the line to the right of
the bridge or power meter should have no effect on
the meter reading, regardless of the quality of
the bridge or power meter. Plate current in the
transmitter should not be affected either. If
changing line length affects any meter reading,
the transmission line is not properly terminated
which means the load is not matched to the line.
The degree of mismatch will be indicated in a
rel*ative manner by the amount of change in meter
readings.

M
Antenna
236" x .66 = 39" 4
144 Mc. 82" x .66 = 13.5" 4
220 Mc. 54" x .66 = 8.8" 4
432 Mc. 27.4" x .66 = 4.5" 4
50 Mc.
It is possible that you will get a reflected
energy reading that is approximately the same when
you insert the quarter wave length line. This is
because you are shifting phase 90 degrees and
perhaps just moving to the other side of the
"hump" in the line. Try different lengths of line,
shorter or longer. Put most faith in whatever
readings make your line look the worst. You cannot
believe SWR values obtained from trans*mission
line type instruments (monimatch) in which the
line is an appreciable frac*tion of a wavelength.
At 6 & 2 this is not a problem, but at 432 it is.
knurled
lock-nut -p
A commercial "line stretcher." This one telescopes
20 cm. while main*taining a constant
characteristic impedance of 50 ohms. It is most
useful at 432 Mc. and above where 1/4 wavelength
is about 17 cm. in air.


SWR bridges and power meters are very valuable
instruments, properly used. End.

Also, found a article by the same, useing a 400
foot run of RG-58 U for a dummy load, terminated
with couple 2 Watt 100 ohm resistors (truth be
known, at 6 meters, and up you dont even need the
resistors! The effect, tho is the same, on ANY
freq, as long as the 1/4 (or the 1/2) wavelengths
apply! Jim NN7K


Owen Duffy wrote:


If the SWR on a line is 1:1 at any point, it is 1:1 at ALL points on
the line.

If the SWR on a low loss line is close to 1:1 at any point, it is
approximately the same at ALL points on the line.

On practical lines at HF and above with practical loads, the SWR
decreases smoothly from the load to the source because of line loss,
the lower the loss, the lower the rate at which SWR falls as you
approach the source.

If lengthening the line increases the SWR at the tx, or decreases it
by more than accounted for by the extra line loss, then something else
is happening.

Owen
an a old VHF'er article from the '60's!
Ill post it (in text format, when I find it-- Jim

On Tue, 05 Sep 2006 02:28:41 GMT, Jim - NN7K wrote:

Owen

Thanks Jim. I will insert comments inline...

Keep in mind that I'am (primarily) VHF/UHF op.
You might find this interesting (- the drawings)!
This from the VHF'er, Nov, 1964-- Jim
ACID TEST FOR A FLAT TRANSMISSION LINE Loren Parks
K7AAD (BTW, Loren runs Parks Electronics, in
Portland, Or. and has given up ham radio long ago!)


The mark of a beginner is blind faith in meters.
Most hams get over believing an "S" meter after a
few years, but very few get to the point of
doubting that SWR or power meter. You ought to try
hooking half a dozen SWR bridges and power meters
in series with your transmission line--all at the
same time. If the line is not pro*perly terminated
you'll probably get as many different readings as
you have meters.

The author is not stating what has happened, what he has observed, but
an opinion on what he thinks may happen.

Assuming that he is talking about the common reflectometer /
directional wattmeter instruments that sample current and voltage in a
very small region to determine SWR, that they are suited to the
frequency and line, and they are of quality...

The fact remains that on practical lines at HF and above with
practical loads, the SWR decreases smoothly from the load to the
source because of line loss, the lower the loss, the lower the rate at
which SWR falls as you approach the source.

Any difference between instruments for direct or indirect measurement
of VSWR placed in a uniform practical line is accounted for by line
loss and instrument error (eg its indication or disruption of the
line). The line loss between adjacent meters in practical coaxial
lines (which is where you are likely to be using an SWR meter) on
practical loads is very low, and so quality instruments should read
almost identically.

To take an extreme example, using RG213 at 1296MHz, and two SWR meters
spaced 0.3m (~1') apart, if the SWR on the line at the one nearest the
source is 3:1, the SWR on the line 0.3m closer to the load is 3.1...
you would probably not see the difference. If on actual test, two
meters showed significant difference, there is instrument error in at
least one of the instruments.

.... lots of confused stuff clipped


SWR bridges and power meters are very valuable
instruments, properly used. End.

Prophetic!

Also, found a article by the same, useing a 400
foot run of RG-58 U for a dummy load, terminated
with couple 2 Watt 100 ohm resistors (truth be
known, at 6 meters, and up you dont even need the

Yes, the input SWR of a 400' o/c stub of RG58 is about 1.1 at 50MHz,
and gets lower with increasing frequency.

resistors! The effect, tho is the same, on ANY
freq, as long as the 1/4 (or the 1/2) wavelengths
apply! Jim NN7K

I don't really understand how the 1/4, 1/2 relate to the dummy load?

Owen
--

On Tue, 05 Sep 2006 03:58:56 GMT, Owen Duffy wrote:

If on actual test, two
meters showed significant difference, there is instrument error in at
least one of the instruments.

Hi Owen,

The same thing will happen if you put a SWR (or Power) meter between
the two mismatched ends of a transmission line. In this case it is
not instrument error, it is systematic (application of the instrument)
error.

73's
Richard Clark, KB7QHC