[Elecraft] how did I calculate the loss?

[Charles Greene]

Hi,

It's another of those bits of folk lore, that a balun is lossy except at 
the design impedance.  I did some extensive loss measurements on a 4:1 
current balun working into 700 ohms at a high reactance instead of 200+j0, 
and found the efficiency was 94% (it was 97% on a 200 +j0 ohm load).  Find 
a place in the open wire line where the SWR is less than 5:1 and place your 
balun there.  My tests are documented at: http://www.njqrp.org/

It is also a bit of folk lore that a coax balun is loss less.  Check 
the  loss of your coax at the SWR of the impedance of the open wire feed 
line (not 600 ohms), and refer to a coax table and see what the loss 
is.  May surprise you.  In fact, it's a trade off between the loss of a 1:1 
current balun consisting of a few turns of coax on a ferrite tordial core 
and an air wound balun.  The loss in the ferrite is low as long as it 
remains below saturation, and it uses less lossy coax.

A balun it does two things.  1.  Keeps current off the outside of the coax; 
and 2. in doing this, transforms the feedline from balance (two parallel 
wires) to unbalance (coax).  This is true whether it is 1:1 or 4:1 or some 
other ratio.

On the other hand, many operators report success by bringing a twin lead 
such as 300 ohm ladder line into the shack and hooking one end to the hot 
side of an antenna tuner and the other side to ground.  The feed line is 
not going to radiate as long as it still is two wires, and if your ground 
is poor enough to cause you problems when using coax with no current on the 
coax, it still will be a problem with a balun and coax, or twin leads.

As far as the twin lead goes, a twisted pair of #26 silver plated, 
stranded, Teflon coated wire makes a fine feed line for QRP operations.  It 
has an impedance of about 200 ohms, is low loss, light weight, and easily 
constructed and cheap enough if you can find the wire on the surplus market.

At 10:32 PM 12/15/2003, John Buck wrote:
>Don Wilhelm nicely stated views similar to mine on the use of baluns.
>It should also be noted that many baluns are very lossy at other than the 
>design load termination impedences.  For this reason I tend to use air 
>wound coils of coax where I absolutely feel the need for a balun with a 
>multiband wire antenna.  I admit that my Force 12 beam antennas use the 
>W2DU type baluns at the antenna end of the coax.
>
>Depending on the antenna, so what if the feedline radiates a bit?  When 
>used with typical compromize wire antennas, feedline radiation may even 
>help in some cases.
>Feedline radiation is not loss if you consider the feedline to be part of 
>the antenna.
>
>For example with an inverted V antenna feed line radiation my just improve 
>the vertical component of the antenna radiation, more like a top loaded 
>vertical.
>
>I have had stray rf in the shack as the result of unbalanced feedlines.
>In that case an airwound  balun in the attic away from the radio has 
>solved the problem.
>I use both the K2-100 and the Orion with an SB220 amplifier and with no RF 
>problems.
>
>73,
>John KH7T
>
>Don Wilhelm wrote:
>
>>Vic and all,
>>
>>I respectfully disagree.  The balance or lack of balance is dictated by the
>>antenna system itself (and the RF will be independent of grounds).  So, if
>>the antenna system is inherently balanced, the currents (or voltages if you
>>prefer) on the feedline will be equal and opposite no matter what the drive
>>is - this is not dependent on a balun or lack of a balun anywhere in the
>>system.
>>
>
>snip
>
>>The real reason for a balun at the feedpoint of an antenna is to keep the
>>current from flowing down the outer side of a coax shield.
>snip
>
>>OTOH, if "RF in the shack" is a problem, a balun at the
>>transmitter (or tuner) output may cure it.
>>
>>73,
>>Don W3FPR
>>
>>----- Original Message -----
>>
>>
>>
>>>I forgot to add...wouldn't you also need a balun at the antenna
>>>feedpoint?  If you connect the feedline directly to the antenna the
>>>currents in the legs of the feedline could be unbalanced and then you
>>>might have significant loss from feedline radiation.
>>>
>>>
>>
>>
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>
>
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73, Chas,  W1CG
K2 462b, 3571