John,
Your pair of 50 ohm coax lines has twice the loss of a single coax line
and the characteristic impedance is 100 ohms, so I would expect its loss
to be much greater than using the 450 ohm line with no lumped
transformers (the feedline itself will act as an impedance transformer,
and a quite efficient one at that.
Can I suggest that you might use a length of 450 ohm (or better yet open
wire line) that is about a quarterwave at 30 meters as a matching
section. That will transform the input impedance to something much
higher, then run 450 ohm line to the shack.
It is not unusual to use 450 ohm ladder line with a widely varying range
of antenna impedances. Even when operated at high SWRs, the ladder line
(when dry) has a very low loss.
If your tuner has a limited range and cannot properly match the
feedpoint impedance of the feedline, adding (or subtracting) a length of
line to the feeder length can bring it into a range that your tuner can
match. I would suggest adding or subtracting in 1/8th wavelength
increments until you get it close to the desired range and then go to
1/16th wavelength additions or subtractions. The feedline acts like an
impedance transformer when it is operated with an SWR other than 1.
73,
Don W3FPR
john petters wrote:
Sorry to take up bandwidth here, but as there are some very knowledgable
folk on list I thought I would run this query.
I've been playing with a balanced feeder for my quad - DJ4VM driven
element cut for 20M.
I have 30M reflector. The impedance is very low on 30M and it does not
like the 450 ohm line.
I got over this by putting a 9:1 step down transformer, which works OK,
but I suspect it may be lossy on the other bands where it is not needed.
I thought it would be a good idea to make a balanced line from two
pieces of RG213 to reduce the ohmic losses of the cable (the run of 450
ohm measures 6 ohms DC) and get a closer chacteristic impedance to the
the quad on 30M
I made two lengths of RG213 coax with the inner and outer of each cable
parralled and the two cables stuck together with insulating tape as a
balanced line This did not work at all well for some reason.
I have also (as per the RSGB Rad Com Handbook and seen nowhere else)
rearranged the cables so that the braids are connected together and
grounded with the two inners providing the balanced line.
On a small version of the DJ4VM quad as a test antenna, which is only a
couple of ohms on 20m, the K2 sounds dead. removing the braid and using
the feeder as a long wire brings the signals way up.
My query is this. Is there an error in principle in using two pieces of
coax as a balanced line, which would imply the handbook is wrong or am I
missing something.
I did wonder if the braid, used in this manner has a different velocity
factor to the inner and whether this could be the problem - or am I
trying to do the impossible?
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