[Elecraft] Re: Home made Sigma-GT5 & KRC2 or SGC?

[Alan WB6ZQZ]

I have the Force 12 Sigma 5, and it is an excellent antenna both 
mechanically and electrically. The tuning system is relays and coils 
effectively adding length to this short vertical dipole with capacitive 
hats on both ends.

The antenna is designed for the highest frequency, and then the matching 
system is designed to handle the lower frequencies. The efficiency of the 
physical size limits the lower end of the frequency range. The Sigma 5 
could be resonated on 30 or 40 meters but is not large enough to have good 
efficiency there (and Force seems to refuse to build any low efficiency 
antennas).

This antenna and matching system design works well over a 2:1 frequency 
range. So if you set up a 20 meter fullsize version you could use the same 
technique to get to 30 and 40 meters. Matching this antenna on 17,15,12 and 
10 meters would not be as simple.

I believe MFJ has a vertical halfwave that will span the 40 to 10 meter 
range. It has more elements than the Force. That is one way to keep the 
matching system simple - add elements for different bands, or band ranges.

Another good setup would be to use two antennas - the 20-10 and a 30 meter 
set up with coils and relays for 40.

The Force antennas are mechanically excellent - simple and strong. They are 
easy to disassemble and take to field. Others like the MFJ are often quite 
complicated and not as suitable for portability - but they pack more bands 
into one antenna.

I also have an SGC 239 tuner. As others have mentioned we really need a 
balanced autotuner for an application like this. I have deployed the SGC on 
many antennas in the field and here at home. It tunes quickly and produces 
an excellent SWR on the inverted L configurations I usually use. 
Unfortunately at power levels above 20 watts it occasionally reboots and 
loses its mind for awhile, which is quite hard on the SWR, signal strength 
and the QSO I was making. After a minute or so it comes back and works 
again. The SGC service is very good at blaming everything but the tuner, 
which I suspect is getting RF into the microcontroller in some antenna 
configurations. I have heard from many others with similar problems. So I 
cannot recommend SGC.

The Icom AH4 works fine (even in situations the SGC has trouble in), and it 
is not hard to build a control box for a non Icom radio. However it, too, 
is an unbalanced tuner.

Using an unbalanced tuner on a balanced antenna (such as a vertical dipole) 
can be problematic. As others have pointed out, we really need a good 
inherently balanced autotuner. In a weatherproof box with a remote control 
panel. Whoever makes a good one will sell a lot of product. MFJ has started 
making a manual balanced tuner, but I'm not aware of any other commercial 
balanced tuners at all! Elecraft, are you listening?

Using a Balun on an unbalanced tuner is often a marginal solution. Bead 
baluns are great if there is no imbalance and the SWR is 1:1. If there is 
actually imbalance they are quite lossy. Jerry Sevick's various Balun books 
cover the subject and he has actually measured the losses in most all balun 
types so you can see how they actually perform. Designs optimized for Tuner 
Baluns are covered in Jerry's "Understanding, Building and Using Baluns and 
Ununs" book. However tuners such as the SGC have such wide impedance and 
mismatch ranges that any Balun design can be extremely stressed and become 
quite lossy, or fail to do its job of forcing balance. The effectiveness of 
a tuner plus balun varies with the situation so just because it works fine 
in one scenario does not mean that it will work well in another. If the 
inherent balance is fairly good and the impedance mismatch is not too great 
it will work fine. If not it can be quite lossy and the lack of balance can 
put RF in places you don't want it and make the system squirrelly.

Another configuration you might want to try before embarking on your big 
project is to use a good tuner-balun (from Jerry's book) right at the rig. 
 From the balun run dual coaxes just long enough to get outside. Use big 
fat coax for this. Then use ladder line or windowed twinlead to the 
antenna. You can even try this on your existing balun. Check for balun 
losses by checking balun temperature rise. The dual coax is not low loss, 
but it will help you get the line out of the shack. Approximately the same 
effect can be had by putting the balun just outside the shack and using 
heavy coax to the tuner and ladderline or windowed twinlead to the antenna, 
if your balun is low enough loss and outdoor compatible.

-- Alan WB6ZQZ

At 01:04 AM 3/21/2004, Jeff wrote:
>From: <jeff_elecraft@burnshome.org>
>Date: Sat, 20 Mar 2004 12:44:05 -0500
>
>I am evaluating ways to improve my antennas. My current arrangement is
>losing 3 to 6 db because I am using the KAT2 to tune a non-resonant antenna
>fed with coax. Routing open wire line is problematic so I am considering
>matching at the feed point.
>
>The Force 12 Sigma-GT5 looks interesting, but I would like to cover 30 and
>40 meters. It should be possible to make a matching network similar to the
>Sigma-GT5 for either the Sigma 40 or 40XK. The KRC2 Band Decoder could
>switch the relays. How hard would it be to make this network and get it
>working? Would I be money and time ahead to just buy a SGC tuner and mount
>it at the feed point?
>
>...
>
>Also, is there anything about the Force 12 Sigma antennas that would be
>difficult to fabricate at home?
...