In a message dated 10/2/01 11:13:34 AM Pacific Daylight Time,
----- Original Message -----
To: <firstname.lastname@example.org>; <email@example.com>
Sent: 01 October 2001 22:05
Subject: Re: [TowerTalk] homebrewing short yagis
>One DE was a FD set for 100 ohms that was fed with balanced 100 ohm coax
into a Johnson Match Box Tuner. It works great on any beam (Quad also).
Would you mind saying a bit more about this, please ? Tim, EI8IC
I'd be glad to and I thought you would never ask hi. I normally enlarge the
font to make it easier to read further away from the screen. TT E-mail
usually reduces it to 10. This came through at 14 font.
A Folded Dipole (FD) can be made for differrent R's regardless what the R is
for a split DE. It was fairly common years ago in the Stone Age of the last
century to add a #12 wire below the larger DE, make a FD out of it and feed
the small diameter wire for a R step up. The spacing was usually around
2-3". It depended on the original R of the split DE. It produced R's around
200 ohms and it could be fine tuned for 200 ohms by adjusting the spacing.
Spacers were used with multiple slots. If changing the spacing didn't
develope the 200 ohms, changing the wire size would with proper spacing
adjustments. The smaller the wire the higher the step up.
A 1/2 wave of coax (using VF) was used to make a balanced 200 ohm feedline.
See the Sone Age Manuals of the last century for how to do it. A 1:1 SWR
means you have a 200 ohm R. This works great with balanced feed from an
unbalanced coax but there is evidence it also reduces bandwidth even feeding
a FD DE which usually increases it. I use this on one of my 6M 5 element
beams. I will be able to show digital pictures of this soon. There used to
be charts in the various Antenna Handbooks in the 40's&50's (maybe sooner)
that gave step up ratios (multiply times the split DE R) for various DE
My favorite is adjusting the 2 FD diameters for a 100 ohm feedpoint. On my
W8CC 5 element 10M beam I used 1.25" (with 1.125" end tips) and .75" tubing
about 2.5" apart. To obtain 100 ohms feed the center of the 1.25" tubing
using old HyGain insulated hardware. The .75" tubing is continious. 100 ohm
balanced coax (2-50 ohm coaxes series connected) requires no balun and is a
great feeline (nothing to saturate either). It's a bit lower loss than a
single coax and a bit more tolerant to SWR. It was 91' 2" long so it was 4
WL long for a variety of reasons. NOTE! I had a SWR bridge and RF ammeters
in each leg to check balance. The Johnson Match Box matches 100 ohms with
very very low loss. I ran a 30L1 linear at 600W into a 275W AM rating Match
Box and it barely got warm. I added another SO-239 female connector.
Although the MB is enclosed, mine or copies of it are often open and with no
RF in the shack. I can get max RF into the input of the feedline all over
the band as the rig sees 50 ohms when the MB is properly tuned. I have 2
calibrated dials with recorded settings across the band. It's so efficient
the only way to improve it is with the KW MB. I also use a balanced L with
both Xc's and rotary XL's on one shaft for a very quick tune up in a unique
way. It's one of my 1 Knob L Network Tuners. These are fairly small silver
plated coils .175" diamter that don't even get warm with 1.5 KW on 10M. The
Xc spacing is fairly close also and no arcing. Another tuner I made using
the MB configuration had 150W plug in coils with 3/16" wire to make it rigid.
These concepts will be published for the doubters or those who actually want
to try it--before doubting.
It's a great system that makes any beam Kick Butt and gives the best possible
pattern which many never see with a Gamma for example. The 100 or 200 ohm
resistive feedpoint really reduces the DE Rlosses to a point where you don't
have to worry about them. You can make a 400-600 ohm DE with multiple wires
or the Delta Match for even lower current in the feedpoint but you will have
to look real close to measure the difference in output. I've done it but it
requires open wire line. It's great for feedline lengths of say 300' or
more. I had a beam in the baseball park back in SD in the late 40's.
The 100 ohm feedline lends itself perfectly for 2 element quads. This is
what I used on the 2 element 6M quad into a modified MB--different coil. I
also used an input link with more turns for a 200 ohm balanced feedline
thereby not grounding the shield to the chassis which just puts RF on the
chassis and serves no useful purpose. The tuner was completely open and I
had no TVI on 6M as I did with every other 6M antennas I tried.
Not many in ham radio ever experience an antenna that really Kicks Butt. The
right feed system can open your "RF Eyes" and justify the rest of your HR
In the 30's 600 ohm slip rings were used to allow the beam to rotate
continiously without feedline damage. The Mim's beam used them also. I'm
going to revive the slip rings for lower Z's and see how efficient it is.
There is a querk I discovered with the 100 ohm balanced feedline and
different tuners. This is a quickie description. There are turns ratio's of
link to tank coil, L/C ratio's and the link series Xc values (if used) that
created strange affects to the output RF. Using a SWR bridge and RF ammeters
in each coax I could get setting that gave 1:1 SWR but would give
1. Balanced currents of the right values and 1:1 SWR in each leg with a 100
ohm load. This is what you expect.
2. Unbalanced currents and 1:1 SWR in each coax. (1:1 input SWR) (No hot
3. Other than 1:1 SWR in each bridge and balanced currents. (1:1 input SWR)
#1 always worked the best.
This occurs with the balanced L, Parallel tuners and perhaps certain MB
configurations also as I remember now. I never would have discovered this
unless I had a SWR bridge and RF ammeters in each coax leg.
Many can't conceive this and I have to demonstrate it to them with money on
the side and I haven't lost yet. Has anyone ran into this before? It may
explain why so many have trouble with certain tuners and noticible losses.
With better measuring techniques things like this show up. Interputing data
properly and taking the right steps can maximize perfomrance with more RF up
the feedline. When I get time I will address this much further and record
all the data. Has anyone else found this situation with balanced coax?
Perhaps if you couldn't get a tuner to work right with open wire line, a
tanget of this was in there screwing it up. I've had unbalanced currents in
open wire line when I thought I shouldn't and the antenna had good physical
balance. I changed the balanced tuner configuration and the currents
balanced up. Don't do this after a few beers.
There is even more to this and other querks I haven't mentioned but solved
and perhaps why I've been able to get tuners to work with no heating into
various feedlines so well. Anyone with constructive comments or related
experiences? Some day I hope to put all this together in a Antenna Tuner
Handbook with things you can duplicate in this New Age--others have. k7gco
List Sponsored by AN Wireless: AN Wireless handles Rohn tower systems,
Trylon Titan towers, coax, hardline and more. Also check out our self
supporting towers up to 100 feet for under $1500!! http://www.anwireless.com
FAQ on WWW: http://www.contesting.com/FAQ/towertalk
Administrative requests: towertalk-REQUEST@contesting.com