[Elecraft] OT:The use of a doublet

[john petters]

Ron D'Eau Claire wrote:
Extremely high currents can
> flow at some points along the line, and large conductors help minimize the
> ohmic losses in the line. For that reason, 300 ohm "twinlead" or 450 ohm
> "ladder line" works, but has larger-than-necessary losses because of the
> small diameter conductors those lines use. Whenever possible, I use a #12 or
> larger wire for my open wire lines. 
 > It will work well down to where the antenna is only 1/4 wavelength
 > end-to-end, showing only a small loss compared to a full size dipole. The
 > problem there is that the SWR on the feeder jumps up as the impedance 
drops
 > quickly at that size.
 >

I noticed an improvement with my DJ4VM quad, which is a balanced 2 el 
design for 20M, but works with gain on the other HF bands. No problem on 
20 and above, but I found that feeding it with 450 ohm twin on 30M where 
the impedance is low was a problem. I got round it by putting a 9:1 
balun between the element and the line.

Now, having beefed up the guage of the element itself, it tunes 30m 
without the balun and I can get 599 reports from the East Coast USA on 
my 10 watt K3. Good f/b also.

It would seem from what you say Ron, that increasing the feeder guage 
may be make it even better.

Another not unexpected result was experienced when using a G4LNA quad 
loop, a vertical square about 25 feet each side for 160M
http://www.qsl.net/g4lna/pages/myant.html
This is fed with 50ohm coax to a step down transformer on a ferrite 
ring. The secondary goes to a series variable C then on to the loop.

Mounted only anout 2 feet above group, this radiates a big signal on 
topband for its size and was only a couple of s points down on my half 
wave doublet.
I thought I'd be clever and ditch the transformer and slap the 450 ohm 
line straight on to the loop and let the balanced tuner handle it.
Guess what. The SWR was perfect - but everything was being lost in the 
line / and or the tuner.

The G5RV is another story. Everyone misuses these antennas. They should 
be fed balanced all the way and they become - a doublet. As most folk 
use them, with a balnced transformer into, hopefully, a balun, then coax 
into a tuner. This is a definate no no.

The transformer can only be right for a given frequency and that will 
change with height above ground, whether the elements are horizontal or 
inverted V etc.

Nothing wrong with a 5RV if fed directly with balanced line - but you 
would be better cutting it as an extended double zepp for 20 and get 
3DBd gain.
73


John Petters
www.traditional-jazz.com
Amateur Radio Station G3YPZ

-- 
John Petters
www.traditional-jazz.com
Amateur Radio Station G3YPZ