Series-Section Xmission Line Impedance Matching
 

Hi,
I would like to cut a 75 ohm coaxial cable to 30 electrical degrees at 7.2
Mhz. for use in a series matching section. The velocity factor of the cable is
not known. I have an Antenna Analyzer, and and RFsignal generator + the usual.

The impedance of the coax is stamped on the jacket, any way to determine 30
degrees without guessing at the VF?
73 Gary N4AST

You could use the antenna analyzer to find out what frequency the whole
cable is 1/2 wavelength at, measure its length, and then calculate the
velocity factor. Personally, I would guess that the factor is no less than
65%, whack off a piece, and take it from there.

Tam/WB2TT
"JGBOYLES" wrote in message
...
Hi,
I would like to cut a 75 ohm coaxial cable to 30 electrical degrees at
7.2
Mhz. for use in a series matching section. The velocity factor of the
cable is
not known. I have an Antenna Analyzer, and and RFsignal generator + the
usual.

The impedance of the coax is stamped on the jacket, any way to determine
30
degrees without guessing at the VF?
73 Gary N4AST

JGBOYLES wrote:
Hi,
I would like to cut a 75 ohm coaxial cable to 30 electrical degrees at 7.2
Mhz. for use in a series matching section. The velocity factor of the cable is
not known. I have an Antenna Analyzer, and and RFsignal generator + the usual.

The impedance of the coax is stamped on the jacket, any way to determine 30
degrees without guessing at the VF?

It's easy to cut a 90deg length, so do that at 21.6MHz.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek

It's easy to cut a 90deg length, so do that at 21.6MHz.

Now why didn't I think of that! Thanks Ian.
73 Gary N4AST

I second Tam's advice...good comments as usual.

In addition, realize that the velocity factor (at HF and above) is
determined practically entirely by the dielectric used in the line.
You can use this fact to at least bracket the reasonable values. If
it's solid polyethylene dielectric, the VF will be very near 0.66. If
it's solid Teflon, VF will be very near .69. It's usual for
foam-polyethylene to give a VF near 0.78, and foam-Teflon closer to
0.82, but since the percentage plastic in the foam affects the VF,
it's a bit more variable than with solid dielectric. In addition, if
you can measure the capacitance and you are reasonably sure about the
impedance of the line, you can use that to get an estimate of the VF.
That's because Zo=sqrt(L/C) and Tau=propagation time=sqrt(L*C).
Knowing C and Zo, you can find L, and from that you can find Tau, and
from that and the length, you can find VF. Assume a tolerance of
+/-5% on Zo...it might even be more.

Depending on the test equipment you have available and the trouble
you're willing to go to, there are other ways you can nail VF down.
It might be worthwhile to do a simple calc to understand how well you
need to know the VF, given that you probably don't know the Zo closer
than 5%. You may decide it doesn't matter a whole lot as long as
you're in the ballpark, or you may decide that you also need to
determine the true Zo as well.

Cheers,
Tom

"Tarmo Tammaru" wrote in message ...
You could use the antenna analyzer to find out what frequency the whole
cable is 1/2 wavelength at, measure its length, and then calculate the
velocity factor. Personally, I would guess that the factor is no less than
65%, whack off a piece, and take it from there.

Tam/WB2TT
"JGBOYLES" wrote in message
...
Hi,
I would like to cut a 75 ohm coaxial cable to 30 electrical degrees at
7.2
Mhz. for use in a series matching section. The velocity factor of the
cable is
not known. I have an Antenna Analyzer, and and RFsignal generator + the
usual.

The impedance of the coax is stamped on the jacket, any way to determine
30
degrees without guessing at the VF?
73 Gary N4AST