[Elecraft] OT- Using two coaxial cables as a balanced feeder

[john petters]

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?

I have experienced a related problem in the past when I used  a smalll 
loop antenna on 160M - about 25 ft on each leg, mounted vertically. The 
botton being about three feet above ground.   The design was by Paul 
G4LNA as can be seen on the link below

http://www.geocities.com/g4lna/pages/myant.html

Fed with 50 ohm coax through a step down transformer with one side of 
the secondary going though a series capactor to one side of the loop, 
the other side of the transformer connected to the return of the loop. 
This antenna worked very well on 160 - but if fed with 450ohm line 
through a balanced ATU, did not radiate at all despite a low SWR.

Any ideas welcome off list.
73
-- 
John Petters
www.traditional-jazz.com
Amateur Radio Station G3YPZ