[Elecraft] Baluns and 450 ohm line

[kc4kgu@EnterZone.Net]

On Tue, 4 Jun 2002 k6se@juno.com wrote:

> John, KD4KGU wrote:
> 
> "The two center conductors are used for the feedline and the shield
> floats at the antenna and is grounded at the shack."
> ==========
> I'm still wondering why you would want to use a 100-ohm feedline to feed
> an antenna which you said your MFJ-269 indicated the feedpoint Z was 50
> ohms at resonance.
> 
> 73, de Earl, K6SE
> 67 countries with K2 #2622 and counting.....

The dipole was a good 50ohm 1:1 match at resonance.  The twin-coax feed
was used on it because I was using it for other bands and the 100ohm
feedline provided better results on the other bands.

I have since built full-wave loops for 160m, 80m, 40m, 30m, 20m, 17m, 15m
and 10m.  The feedpoint impedence of these full-wave loops is
100ohms.  Since I won't be operating them on bands other than what they're
cut for, I wanted to match the impedence of the feedline as closely to the
impedence of the antenna as possible.

I intend to employ a remote coax switch to select the appropriate antenna.  
I'll then be running a run of RG214/U from the shack to the remote antenna
switch.  From the antenna switch, for each antena, it will be a short run
of RG8/X to a 2:1 balun and then the appropriate length (1/2 f) of
parallel RG8/X from the balanced side of the 2:1 to the feedpoint of the
antenna.

Something like so:

[SHACK]====[RG214/U]====[Remote antenna switch]
                             |
                             |  <- 50ohms (RG8/X)
                             |
                           [2:1]
                            | |
                            | | <- 100ohms (dual RG8/X)
                            | |    1/2 f in length
                        +---+ +---+
                        |         |
                        |Full-Wave|
                        |Loop     | <- feedpoing = 100ohms
                        |         |
                        +---------+

Depending on the size/weight of the 2:1 balun I come up with for the
antennas, they may be mounted directly at the antenna feedpoint.  I'm not
certain yet.

The RG8/X may be more expensive than ladderline or open-wire line but, it
is MUCH less visible and extremely forgiving as far as proximity to metal
objects, the ground, etc.

Does this sound like a good idea, bad idea, strange idea?

73 de John - KC4KGU
K2/100 #2490