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

[Don Wilhelm]

Folks,

I just received a correction from a reliable source (Tom W8JI).  The 
loss in a parallel set of coaxial lines is not 2 times the loss for a 
single line, but the same as a single line.

I believe my other points still hold true.

73,
Don W3FPR

Don Wilhelm wrote:
> 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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