[Elecraft] how did I calculate the loss?
[email protected]
[email protected]
Tue Dec 16 11:11:01 2003
I too connect the 300 ohm twin lead directly to my KX1 without a balun. =
Now there will be feedline radiation doing this. If anyone wants to see =
a simulation just email me and I'll email you a word document showing =
the example of a 20 meter dipole fed by open line. In one case it is fed =
differentially and there is no feedline radiation (as shown by the =
polarized radiation patterns and the wire currents). In the second case =
it is fed with a single-ended source. The feedline radiation can be =
clearly seen, although as pointed out by others, this is not necessarily =
a bad thing.
Dave=20
-----Original Message-----
From: [email protected]
[mailto:[email protected]]On Behalf Of Charles Greene
Sent: Tuesday, December 16, 2003 4:53 AM
To: John Buck; [email protected]
Subject: Re: [Elecraft] how did I calculate the loss?
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=20
current balun working into 700 ohms at a high reactance instead of =
200+j0,=20
and found the efficiency was 94% (it was 97% on a 200 +j0 ohm load). =
Find=20
a place in the open wire line where the SWR is less than 5:1 and place =
your=20
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=20
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=20
is. May surprise you. In fact, it's a trade off between the loss of a =
1:1=20
current balun consisting of a few turns of coax on a ferrite tordial =
core=20
and an air wound balun. The loss in the ferrite is low as long as it=20
remains below saturation, and it uses less lossy coax.
A balun it does two things. 1. Keeps current off the outside of the =
coax;=20
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=20
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=20
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=20
is poor enough to cause you problems when using coax with no current on =
the=20
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,=20
stranded, Teflon coated wire makes a fine feed line for QRP operations. =
It=20
has an impedance of about 200 ohms, is low loss, light weight, and =
easily=20
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=20
>design load termination impedences. For this reason I tend to use air=20
>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=20
>the antenna.
>
>For example with an inverted V antenna feed line radiation my just =
improve=20
>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=20
>solved the problem.
>I use both the K2-100 and the Orion with an SB220 amplifier and with no =
RF=20
>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=20
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