I want to measure and cut some coax for 14.200 MHz. It is to be connected
to my Cushcraft MA5B
I have cut off a 100 feet of RG-213 coax and have put a PL-259 on one end.
I have a T connector connected to the analyse with a 50 Ohm dummy load on
one side of the T and the 100 feet of coax on the other side of the T.
I then go into SWR mode on the Analyser and sweep, the nearest dip it gives
is at 13.105 MHz. I expected it to be nearer to 14 MHz. Would this be about
right?

Before I start cutting bits off the coax I wanted to make sure that this is
correct or is there is a better way?

How crucial is the length of coax in the first place?

73 Andy

"Andy" wrote in message
...
I want to measure and cut some coax for 14.200 MHz. It is to be connected
to my Cushcraft MA5B
I have cut off a 100 feet of RG-213 coax and have put a PL-259 on one end.
I have a T connector connected to the analyse with a 50 Ohm dummy load on
one side of the T and the 100 feet of coax on the other side of the T.
I then go into SWR mode on the Analyser and sweep, the nearest dip it
gives is at 13.105 MHz. I expected it to be nearer to 14 MHz. Would this
be about right?

Before I start cutting bits off the coax I wanted to make sure that this
is correct or is there is a better way?

How crucial is the length of coax in the first place?

73 Andy

Since the MA5B instructions say that the feeder length is not important, why
do you need to cut the feeder to a particular length??

Jeff

"Jeff" wrote in message
. com...

"Andy" wrote in message
...
I want to measure and cut some coax for 14.200 MHz. It is to be connected
to my Cushcraft MA5B
I have cut off a 100 feet of RG-213 coax and have put a PL-259 on one
end.
I have a T connector connected to the analyse with a 50 Ohm dummy load on
one side of the T and the 100 feet of coax on the other side of the T.
I then go into SWR mode on the Analyser and sweep, the nearest dip it
gives is at 13.105 MHz. I expected it to be nearer to 14 MHz. Would this
be about right?

Before I start cutting bits off the coax I wanted to make sure that this
is correct or is there is a better way?

How crucial is the length of coax in the first place?

73 Andy

Since the MA5B instructions say that the feeder length is not important,
why do you need to cut the feeder to a particular length??

Jeff


with velocity factor of .66 for generic rg-213 i get 12.988MHz being 2
wavelengths... but why do you care?

here is how to calculate the right length, no fancy measurement equipment
needed.
http://www.qsl.net/w4sat/howlong.htm

Hi Andy

I think you'll will get into a large discussion over this!

I am no means an expert on this topic and everytime I need to be, I have
to relearn a few things. In my experience though the almost only time I
want to cut coax to length is if I am using it for Z transformation
between antenna and TX. Like if the antenna has a Z of 100r I will end
up using odd quarter wave multiples of 75r coax to "match" the system. I
also use it to cut stubs for notch/band pass filtering and tuning . For
one project I remember using a quarter wave length across the 5V
terminals of an old SMPS to reduce radiation on about 75MHz.

What I am getting at is if the TX source Z is 50r and the antenna load Z
is 50r any length of coax will do. The effect of tuning the coax length
will be most evident if the system is reactive at either end. How much
that difference it really makes I dont have experience in. I tend to shy
away from coax systems in this case anyway as high VSWR on the line
itself is a recipe for a large loss of system efficiency. (ie it has
become part of the system tuning and can be regarded as a lossy load)

I normally use resonance/dip oscillator to determine length. If you only
have a VSWR device you can figure the length by actually leaving the far
end open or short and actually going for a maximum rather than dip.

I apologize for not giving you a definitive answer.

Cheers Bob VK2YQA

Andy wrote:
I want to measure and cut some coax for 14.200 MHz. It is to be connected
to my Cushcraft MA5B

Hi Dave

One of the pitfalls is that although a VF of .66 (or .87 for foam) is
common it can vary considerably.

I dont think I have ever seen manufacturers VF specs with a tolerance
listed. In fact I dont think I have ever seen a manufacturers spec
include VF!

Of course it depends on how critical the application is. In my
experience for example 4:1 coaxial baluns on 2m always need to be tuned...

Cheers Bob VK2YQA

Dave wrote:
f


with velocity factor of .66 for generic rg-213 i get 12.988MHz being 2
wavelengths... but why do you care?

here is how to calculate the right length, no fancy measurement equipment
needed.
http://www.qsl.net/w4sat/howlong.htm

Of the two questions, how to do the 'finding the right length of feed
line', and 'is it important', the last question sort of takes
precedence. It just isn't that important at all.
The first question. It's been a while since I've looked at the
instruction manual, but I think the analyzer's book should tell you
how to go about it. (I'm too lazy to look, so take that for what it's
worth.)
- 'Doc

right, and for impedance transformers and stubs you have to take that into
account and tune them individually. I do that for my stub filters here.
But this is a case of how long to make it from the antenna to the radio from
what little he told us, so vf is not important.

"Bob Bob" wrote in message
...
Hi Dave

One of the pitfalls is that although a VF of .66 (or .87 for foam) is
common it can vary considerably.

I dont think I have ever seen manufacturers VF specs with a tolerance
listed. In fact I dont think I have ever seen a manufacturers spec
include VF!

Of course it depends on how critical the application is. In my
experience for example 4:1 coaxial baluns on 2m always need to be tuned...

Cheers Bob VK2YQA

Dave wrote:
f


with velocity factor of .66 for generic rg-213 i get 12.988MHz being 2
wavelengths... but why do you care?

here is how to calculate the right length, no fancy measurement equipment
needed.
http://www.qsl.net/w4sat/howlong.htm

" What I am getting at is if the TX source Z is 50r and the antenna load Z
is 50r any length of coax will do.

Yes, of course, (neglecting any loss in the cable).

The effect of tuning the coax length
will be most evident if the system is reactive at either end. How much
that difference it really makes I dont have experience in.


It makes no difference to the vswr, all changing the cable length will do is
change its phase.
(again neglecting loss in the cable, and assuming that the cable impedance
matches the source).

73
Jeff

Bob Bob wrote:
In my experience though the almost only time I
want to cut coax to length is if I am using it for Z transformation
between antenna and TX.

Unfortunately, when SWR 1:1, 50 ohms is at the
center of the SWR circle and is the target impedance
to which it is impossible to transform (unless feedline
losses are very high). :-)
--
73, Cecil http://www.w5dxp.com

"How crucial is the length of coax in the first place?"
As long as all the impedances are matched, and they are with RG-213 and a
MA5B, length is irrelevant except that as-short-as-possible minimizes
resistive losses.

73, H.
NQ5H

"Andy" wrote in message
...
I want to measure and cut some coax for 14.200 MHz. It is to be connected
to my Cushcraft MA5B
I have cut off a 100 feet of RG-213 coax and have put a PL-259 on one end.
I have a T connector connected to the analyse with a 50 Ohm dummy load on
one side of the T and the 100 feet of coax on the other side of the T.
I then go into SWR mode on the Analyser and sweep, the nearest dip it
gives is at 13.105 MHz. I expected it to be nearer to 14 MHz. Would this
be about right?

Before I start cutting bits off the coax I wanted to make sure that this
is correct or is there is a better way?

How crucial is the length of coax in the first place?

73 Andy