Quarterwave line
 

What would be the best way to adjust a quarter wave of 75 Ohm twin
line using an antenna analyser? I just connected mine across the
analyser (open circuit) and looked for the freq where a short showed -
is that likely to be accurate? At 10.1 MHZ I ended up with a 15.7 foot
line which seems a bit short (low velocity factor).

73

Richard
G3CWI

Richard,

Interesting situation. You are short by a couple of inches from what one
would expect. I wonder if you have to take into account the few inches of
line inside the meter. Does the manual say anything about that? You should
certainly count the length of any connectors/adapters as part of the line
length.

Here is something else you could do. Connect a 50 Ohm, or so resistor from
the MFJ to the cable. Then a diode (1N914, etc) from that junction to a DC
voltmeter. Now adjust the frequency for minimum DC voltage. You may have to
interpolate between the two points where the meter goes to 0. If it tunes
too broadly, make the resistor 0. I have done this with a 'scope, but not a
voltmeter,

Tam/WB2TT

"Richard" wrote in message
om...
What would be the best way to adjust a quarter wave of 75 Ohm twin
line using an antenna analyser? I just connected mine across the
analyser (open circuit) and looked for the freq where a short showed -
is that likely to be accurate? At 10.1 MHZ I ended up with a 15.7 foot
line which seems a bit short (low velocity factor).

73

Richard
G3CWI

On 3 Oct 2003 07:54:50 -0700, (Richard) wrote:

What would be the best way to adjust a quarter wave of 75 Ohm twin
line using an antenna analyser? I just connected mine across the
analyser (open circuit) and looked for the freq where a short showed -
is that likely to be accurate? At 10.1 MHZ I ended up with a 15.7 foot
line which seems a bit short (low velocity factor).

73

Richard
G3CWI

Hi Richard,

I believe your data may be correct. Electrical length Lamda at 10.1 MHz is 97.24
feet, and Lambda/4 is 24.35 feet. And 15.7/24.35 = 0.645 for the VF. Although
this VF is in the vicinity of what you'd find in coax, if there's a lot of
dielectric surrounding the conductors, this value could be correct.

Suggest you check the impedance at 20.2 MHz to see if the input impedance goes
to a very high value, as one would expect for an open-circuited half wave. In
addition, if you record the actual resistance values for both measurements and
take the square root of the product you will have the Zo of the line. If it
comes out close to 75 ohms it will prove the measurements are valid.

Hope this helps,

Walt Maxwell, W2DU

Richard, G3CWI, wrote:

What would be the best way to adjust a quarter wave
of 75 Ohm twin line using an antenna analyser? I just
connected mine across the analyser (open circuit) and
looked for the freq where a short showed - is that
likely to be accurate? At 10.1 MHZ I ended up with a
15.7 foot line which seems a bit short (low velocity
factor).

With an open-circuited stub, there is always a bit of
fringing capacitance around the open end which will
throw the measurement off a bit. A short-circuit
measurement is generally more accurate.

Jim, K7JEB

V.F.=.64 does seem a bit low, but if it's the type of line where the
wires are very close together, embedded in thick, solid polyethylene,
it's not too far out of line. It's easier to get a good short than a
good open, because of fringing effects, but the error from that should
be very tiny with small line at that frequency. If your analyzer
finds high impedance, too, try shorting the other end of the line and
see if you get the same result. Or look for a half-wave reflection of
the short back to a near-short again. Also, check the analyzer with a
physical short right at the analyzer. Is the analyzer for unbalanced
lines? If so, see if a simple ferrite bead choke balun on the line at
the analyzer makes a difference in the measurement.

Cheers,
Tom

(Richard) wrote in message . com...
What would be the best way to adjust a quarter wave of 75 Ohm twin
line using an antenna analyser? I just connected mine across the
analyser (open circuit) and looked for the freq where a short showed -
is that likely to be accurate? At 10.1 MHZ I ended up with a 15.7 foot
line which seems a bit short (low velocity factor).

73

Richard
G3CWI

What would be the best way to adjust a quarter wave of 75 Ohm twin
line using an antenna analyser? I just connected mine across the
analyser (open circuit) and looked for the freq where a short showed -
is that likely to be accurate? At 10.1 MHZ I ended up with a 15.7 foot
line which seems a bit short (low velocity factor).

I believe your data may be correct. Electrical length Lamda at 10.1 MHz is 97.24
feet, and Lambda/4 is 24.35 feet. And 15.7/24.35 = 0.645 for the VF. Although
this VF is in the vicinity of what you'd find in coax, if there's a lot of
dielectric surrounding the conductors, this value could be correct.

I believe I recall reading an admonition (in a grid-dip or analyzer
manual) that measurements of open-circuited stubs were rather tricky
to do accurately. There's always some amount of capacitive coupling
at the open end of the cable which leads to it being less than a
perfect open circuit, and thus won't necessarily look like a perfect
short at the other end of a quarter-wave line. This can throw off the
calculations a bit.

Might be better to short the far end, measure at twice the desired
frequency, and trim until you find the short being perfectly mirrored
at the end of the resulting half-wavelength line.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Thanks for all the comments. I did it open and shorted and averaged
the results (very similar). The cable measured a VF of 0.63 and Z of
120 Ohms - was described as 75 Ohms. The difference caused a big
mismatch and an hour of head scratching and calculator button
pressing.

73

Richard
G3CWI

Tarmo Tammaru wrote:
Interesting situation. You are short by a couple of inches from what
one would expect. I wonder if you have to take into account the few
inches of line inside the meter. Does the manual say anything about
that?

Recent MFJ manuals recommend using an external coax T-adapter with a 50
ohm load on one arm, and looking for the dip in SWR reading
(short-circuited quarter-wave, or open-circuited half-wave). That does
not require accounting for any extra length inside the meter, only
inside the T-adapter itself.

You should certainly count the length of any connectors/adapters as
part of the line length.

Certainly. If you don't like math, then plan to install the resonant
line in the same way as you tested it, using a T-adapter.


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