Cutting Phasing lines to length
In order to feed 4 antennas I needed to cut four lengths of Heliax coaxial cable to equal electrical lengths . These cables were to go from the 4 way power divider to my antennas. I needed each cable to be approx 8 metres long to reach. Note there is no 'magic' length required for these cables sincewe are dealing with a totally 50 ohm impedance system throughout. The transformation of impedance takes place in the 4 way half wave power divider used in the system. The cables have only to be long enough to reach the antennas and of equal lengths.
The way I measured and cut my phasing cables was to use a signal generator connected to an RF Millivoltmeter with an N type T piece adaptor. The Heliax cable to be trimmed was then connected to the the 3rd port of the T piece.
I made the first piece of Heliax (to be used as the reference piece) to the length I wanted it to be (which was about 8 metres). I then tuned the signal generator until I found the first quarter wave 'dip' on the RF Power meter.
In my case this was 8.364MHz.
I continued to tune up in frequency and found the next odd number of quarter wave dip at 3/4 wave. That was at 25.1MHz.
You will get dips at odd multiples of quarter waves 1,3,5,7,9 etc
So I found dips at 8.364, 25.1, 41.9, 58.67, 75.54, 92.33, 109.10, 125.85, 142.677 MHz.
Now I cut my next piece of Heliax a few feet longer than my reference piece. I found the first dip which was less than 8.3MHz and started to carefully cut of a few inches to see how much the dip moved. Once I got closer to 8.3 MHz I started to look for the VHF dip near 142MHz. (The accuracy of the dip is greater at the higher frequencies). I ended up making about 20 cuts before I got the 2nd piece of Heliax to dip at as close to 142.677 Mhz as I could make it. The final few cuts were about 1/2" or less. (About the spacing of the 'rings' in the Heliax outer.)
I repeated the excercise for the remaining pieces of Heliax. Of course I got better at judging how much to cut off each time and got it down to about 8 cuts or so. Its important to keep notes of the dip frequency at each cut and keep the pieces you cut off in order to see how much or little you need to cut to reach your target frequency.
Then it was only a matter of trimming the outer to the correct size to fit the connectors to the other end of the cables.
David Anderson GM4JJJ 1998