FW: [Antennas] Electrical length

[email protected] [email protected]
Thu, 11 Jul 2002 06:25:14 -0700 

George/W5YR
Thank you for your reply.
So to do this ( my project is to make phasing lines for 144Mhz ) then is
to calculate 1/4 wave length 144/246 *(velocity factor)= length in feet.
(I'll be using 5/4) So do I then try to trim for electrical length or
should I leave extra for trimming? 
Is my MFJ 259 accurate enough for this? 

Randy
AC7NJ

-----Original Message-----
From: George, W5YR [mailto:[email protected]] 
Sent: Thursday, July 11, 2002 6:05 AM
To: [email protected]
Cc: [email protected]
Subject: Re: [Antennas] Electrical length

Randy, the classical approach to this is to put an analyzer at one end
of
the cable and find the frequency of lowest impedance. This for the cable
open-circuited at the far end. If the cable is shorted, then look for
the
frequency of highest impedance. This will be the frequency for which the
cable is a quarter-wavelngth long.

With an open cable, the frequency of highest impedance indicates the
electrical length of a half-wave line. Of course, appropriate even and
odd
multiples of these lengths will give similar results.

With an SWR meter, one looks for the frequency of highest SWR for an
open
cable or a shorted cable since either provides a reflection factor of
unity.

I am not aware of a procedure that shorts both ends of the coax unless
they
have in mind dipping a small loop formed in the short at one end.

73/72/oo, George W5YR - the Yellow Rose of Texas         
Fairview, TX 30 mi NE of Dallas in Collin county EM13qe   
Amateur Radio W5YR, in the 56th year and it just keeps getting better!
QRP-L 1373 NETXQRP 6 SOC 262 COG 8 FPQRP 404 TEN-X 11771 I-LINK 11735
Icom IC-756PRO #02121  Kachina 505 DSP  #91900556  Icom IC-765 #02437


[email protected] wrote:
> 
> How do I find (measure) the electrical length of a piece of coax I
have
> the Antenna Book it has me short the center conductor to the shield at
> both ends. The velocity factor is printed on the coax. NOW WHAT?
> 
> I have a MFJ 259 & Dip meter coils for it still in the box
>