[Elecraft] Antenna Theory

[George, W5YR]

Re 4):

Or quite simple if the balanced line is connected to the coax which has, in
my case, five ferrite #43 beads on it, each 1.125" in length. The measured
loss of such a balun was less than 0.1 dB which is in the range of
experimental error. An unbalanced tuner with coax on the output and the
ferrite beads in place forms an efficient transition to balanced line. I use
this on three ladderline-fed antennas on 80-10 with sufficient efficiency to
have done rather well in the QRP-L Fox Hunts the past few years running 5
watts.

Even at QRO, bead baluns are effective and are required to deal only with
the common-mode current on the outside of the coax braid, not the full
differential-mode load current.

Even more effective are coax/toroid baluns made by using coax to wind turns
around a toroid. The advantage over the bead approach is that each bead adds
only its incremental inductance and resistive loss to the balun, whereas the
inductance of the toroid balun increases with the square of the number of
turns.

I use one small balun made from a #43  ferrite bead about 0.5" long with
three turns of RG-174 and it has as much inductance as the five-bead balun
described above. It makes a very effective balun for QRP operation.

These baluns are effective over the 80-10 meter range.

I have a slight case of heartburn over 2), as well. A feedline may have a
large SWR and as long as the impedance presented at the feedline input is
within the efficient range of the tuner, its loss will be small to
negligible.

Line input impedance and SWR are not directly related in this case. SWR is
determined only by the relationship between feedline Zo and antenna driving
point impedance. The SWR seen at the tuner output is modified from that at
the antenna by any feedline loss.

The feedline input impedance depends upon the feedline length, Zo,
frequency, loss, and the impedance with which it is terminated. SWR is
indirectly involved in the sense that input impedance is a function of Zo
and termination impedance which determine SWR, but line length and frequency
are the predominant factors here, not SWR per se.

73, George W5YR
Fairview, TX
w5yr@att.net
http://www.w5yr.com


----- Original Message ----- 
From: "Vic Rosenthal" <vic@rakefet.com>
To: "Mike McCoy" <mikemccoy@austin.rr.com>
Cc: "Elecraft" <elecraft@mailman.qth.net>
Sent: Thursday, March 25, 2004 11:31 AM
Subject: Re: [Elecraft] Antenna Theory


> Mike McCoy wrote:
>
> > Since there seems to be no clear advantage to using coax (other than the
> > fact virtually every piece of equipment made has a SO-259 on the back)
why
> > then is coax THE most prevalent feedline in actual use?
>
> 1) It is unaffected by nearby conductors.  It can be run through metal
> conduits, taped to a tower, even buried (if you get the right kind of
> jacket).
>
> 2) An unbalanced tuner is much simpler and cheaper than a balanced
> tuner.  If the SWR is moderate, then losses are acceptable.
>
> 3) An unbalanced output circuit in a transmitter, especially a multiband
> transmitter, is much simpler than a balanced one.
>
> 4) Given 2 and 3 above, there is likely a need for a transition from
> unbalanced to balanced -- and as many have pointed out, this can be
> difficult to achieve efficiently in a multiband system.
>
> -- 
> 73,
> Vic, K2VCO
> Fresno CA
> http://www.qsl.net/k2vco
>
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