The following discussion about how to calculate the phasing lines took place in
the Moon-Net reflector.
I found the subject so interesting that I decided to put together all the messages in this
page.
On 27 November 1998 KC7YVZ wrote:
I know most people recommend odd 1/4 wave lengths for phasing lines but my lines are to
short, because I didn't have a meter and only measured them with a tape and they ended up
being a 1/4 wave on 148mhz, so I must use 1/2 wave lengths. Is this going to really be
that big of a problem or do I need to look at cutting new ones?
On 27 November 1998 GM4JJJ wrote:
There is nothing magic about using any particular length in wavelengths of phasing lines
if you are using a standard 50 ohm power divider with 50 ohm phasing lines and 50 ohm
antennas.
Provided the lines are equal lengths they will work.- On 144MHz you can get away with just
using equal PHYSICAL lengths of phasing lines (assuming you are using the same velocity
factor of cable for all cables). The reason people use multiples of 1/4 wavelengths is to
allow a method of measuring the line's ELECTRICAL length.
Note that you can still measure your lines electrical length even though they are not a
multiple of your operating frequency, provided you have a signal generator.
The way I did it was to use a signal generator connected to an RF Millivoltmeter with an N
type T piece adaptor. The Heliax cable was then connected to the the 3rd port of the T
piece.
+------------+
|Signal |
|Generator |
+------------+
|
|
/---\ -+-+
| | | | N Type T Piece
| |--------+ |
| | | |
\---/ ++-| Connector
RF | on one end of
Millivoltmeter | Heliax
|
|
|
|
|
| Heliax
|
|
|
|
|
|
|
|
|
| Open End of cable
Trim cable here
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 . That is 3/4 wave. That was 25.1MHz. You get dips at 1,3,
5,7,9 etc quarter waves.
So I got 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 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.
The only mistake I made was to over estimate the length of cable I needed! - I thought
that 8 metres would be right, but in fact I could have used 6.5 metres. The extra loss
will be insignificant, it just meant I had to loop the extra cable a little on the
stacking frame. There was NO way I was going to take those connectors off again and redo
all the work! I could just imagine how stupid I would feel if I got it wrong and I ended
up with the cables too SHORT!!
I guess if you don't have a signal generator and RF millivoltmeter you could use a
transmitter and a power meter with dummy load as the detector.
I've never done this before either, in the past I just measured the cables to the same
lengths physically. In practise that is probably OK with good cable from the same reel at
144MHz. However this time I thought I would try doing it the 'right' way.
On 27 November 1998 N7WS wrote:
I'm guessing that you are talking about a set of equal length lines to go from a power
divider to the feed points of the individual antennas. If this is the case, there is
no requirement for a particular length, as long as they are equal.
The "quarter-wavelength" idea stems from the proposition that for equal
*voltages* at the common point, you get out equal *currents* at the other end. This
ensures equal currents into the antennas even if their feedpoint impedances are slightly
different. With identical antennas, this is hardly a problem.
Furthermore, the usual power divider already has used the "quarter-wave line"
technique, so if you subscribe to this idea, then you really want half-wavelength
"phasing" lines.
So, as in many cases when the question, "How long should the feedline be?" is
asked, the answer is, "Long enough to reach the antenna."
I'll probably get flack on this, but it's my opinion.
(Note: I'm not showing any E-Mail
address here in order to avoid them from being collected by SpamBots. You can
possibly find the E-Mail addresses of the above OM at QRZ.COM.)
This discussion is still not closed. If you have any opinions or additional
related information you would like to be published here, just send
me an E-Mail
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