[Elecraft] RE:balanced tuner

[Jim Brown]

On Mon, 18 Aug 2008 10:29:21 -0500, list1 wrote:

>I have done a lot of EZNEC dipole simulations and have come to the
>conclusion that the loss in 100 feet of coax off of resonance is 
about 
>12 dB per 100'.

"Off resonance" is a very broad description. Loss in coax only begins 
to increase drastically when the SWR is VERY high. There are equations 
(and graphs of those equations) in both the ARRL Handbook and the ARRL 
Antenna Book showing that for MODERATE mismatches, the additional loss 
due to mismatch is rather small. There is also a computer program that 
comes with the Antenna Book that allows you to compute that loss for 
most common coaxial lines and any termination impedances. I have done 
that, and plotted the results.  

Your statement is true only if you're trying to use a single antenna 
to cover all frequencies. It is NOT true if you are using an antenna 
that is close to resonance on the bands you want to cover. I have two 
80/40 FAN dipoles at right angles to each other, fed with about 120 ft 
of RG11. The loss in that line is less than 1.5 dB over all of 80 and 
40M. 

By some form of luck, both of these antennas also come close enough to 
75 ohms on 30M, 17M, 15M, 12M, and 6M that the SWR at the transmitter 
end is less than 3:1. Now, the loss in any line reduces the SWR over 
its length, so a measured 3:1 may be more like 5:1 at the antenna. But 
that still doesn't translate into anything like 12dB of loss!

While I have half-wave dipoles for 30M and 6M, the 80/40 dipoles are 
my only antennas for 17M and 12M, and work well on all of those bands, 
and much better than the resonant dipoles for 30M and 6M. What do I 
mean be "work well?"  If there's a DXpedition and I pileup, I can work 
them, often on the first call. 

My primary objection to balanced line is that it cannot be choked to 
kill receive noise coupled to the antenna by common mode current. This 
current is caused by imbalance in the ANTENNA, which in turn is caused 
by coupling to objects NEAR the antenna (trees, towers, buildings, the 
earth). The world's best balanced tuner and the world's best balanced 
line will not make a dent in this common mode current!  It must be 
killed by a choke, you need choking Z on the order of 5K ohms, and it 
must be at the feedpoint (that is, up in the air). To do that and 
handle serious power, it's EASY with coax. It's hard any other way. 

73,

Jim Brown K9YC