[TenTec] Hot Dang - Rick is a Best Link of the Week on DX-zone.com

Mon Mar 2 11:40:51 EST 2015

NOTE:  This is Off Topic (OT) and probably will be long, so if you are not
interested in antennas and antenna tuners, just exit and delete now.

Jim (W6OGC),

You asked:  "What "matchbox" is the right kind, for 100 watt or less use?"

In order to answer your question, I would have to make some assumptions
because you didn't give us enough information.

"Which matchbox technology" is not a question of how much power you are
running but rather what are the characteristics of the antenna(s) you want
to match with it?  Different types of antennas may have very different types
of characteristics (i.e., impedance, consisting of resistance and
reactance).

If I may assume you are referring specifically to the antenna I described in
the link DX-zone liked, then I can answer it.  If your question was just a
general question, then I will need more information to answer it.

First of all, it may not initially be apparent to everyone, but this is
basically not a high impedance antenna so forget the idea of using a 4:1
balun.  Sure the feedline is 450 Ohms but that doesn't mean the impedance at
the matchbox will be anything like that.

This dipole is 60% the length of a 40m dipole so on 40 and 80m its impedance
is going to be very low.  In fact on 80m it may even be less than 10 Ohms.
Inserting a 4:1 balun between the matchbox and the feedline would
effectively transform that to just 2.5 Ohms - below the point that any
common matchbox could match.

I intentionally chose a non-resonant length (i.e., not an exact half
wavelength on any ham band), so that the even harmonic bands would not have
sky-high impedances.  Of course the length of the feedline will also impact
the impedance, but the chances of avoiding these ridiculously high
impedances are better when beginning with a non-resonant antenna.

Finally, a vertical dipole having one half closer to the ground than the
other is more likely to have common mode current issues than a horizontal
dipole.  In addition, in portable installations, I almost always end up with
the feedline running diagonal (towards the ground), rather than
perpendicular to the antenna.  In this case it is imperative to twist the
feedline often (2 turns per meter if it is the Wireman-like windowline) or
as often as you can without creating a short if it is made of bare openwire.

The best antenna tuner (matchbox) for matching this type of antenna is a
link-coupled symmetrical matchbox, such as the old Johnson Viking Matchbox
(or "Annecke", the German equivalent of the Johnson).  The galvanic
separation of input and output side helps reduce the common mode current.

Unfortunately these link-coupled matchboxes are no longer built - to my
knowledge, nobody in the world is building them, and there are not enough of
them on the used market to fulfill the demand.

The next best choice would be to use a different kind of symmetrical
technology, such as a symmetrical-L (i.e., the Palstar BT-1500A) or
symmetrical-T (i.e., MFJ-974B).  Probably good as well is the S-Match
design, by PA0FRI.  I have just finished building one of these but have not
tested it yet.  I'm waiting on better weather.  Until I have actually used
it myself, I will withhold judgement

None of the above are cheap... except building your own S-Match.

The next step downward is using an asymmetrical matchbox (e.g., T-Network,
L-Network, or Pi-Network) together with a balun between the matchbox and the
openwire feedline.   HERE IS WHERE MOST PEOPLE MAKE A MISTAKE.

Common sense "attempts" to tell us that a 4:1 balun would be better.
Unfortunately common sense is wrong in most cases.  There are two reasons
for this.  First, often the impedance is not as high as one thinks (like
with this antenna), and, in cases where it is extremely high, although the
4:1 seems like a better choice for transforming the impedance downward, it
has trouble dealing with common mode current if the SWR is still more than
about 3 or 4:1.  On the other hand, a 1:1 Guanella balun with enough ferrite
material will cope quite nicely with the common mode current and impede it
as baluns are supposed to do.

G3TXQ and W8JI both do a better job of explaining this than I do, so if
interested, see their respective web sites.  Once you decide to do it, you
can see K9YC's paper on RFI-Ham for more info.

Finally, when trying to cover "all bands", you will encounter a very wide
range of impedances so the impedance matching range of the antenna tuner is
important.  Most auto tuners match about a 10 to one impedance which means 5
to 500 Ohms.  Many built-in auto-tuners in transceivers match only 5:1 and
some will match only 3:1.  It makes a big difference which auto tuner you
choose.

In general, the manual tuners have a much broader matching range than the
auto tuners.
However, when working at home, if you are willing to play with feedline
lengths, it is generally possible to [finally] find a convenient length that
a good auto-tuner can match on all bands.

For "Portable Use", I prefer the manual tuner so that I don't have to worry
about playing with feedline lengths.
Sometimes I still have to adjust the length of the feedline, but not nearly
as often as when trying to use an auto tuner.

Where Power makes a difference is in the presence of heavier common mode
current.
I have never had any noticeable problems with common mode current when
running QRP.
I'm sure the CMC was present, but the level was too low to be troublesome.

Running 100w, you can begin to incur problems that do make trouble so you
have to consider the impedance of common mode current when choosing your
matchbox and balun combination.

Running 500w or more can sometimes incur serious trouble with common mode
current.
If you are going to run high power, especially very high power, you should
study up on how to deal with this and the best place is Jim's (K9YC) paper
on RFI-Ham.
http://audiosystemsgroup.com/RFI-Ham.pdf 

Jim (W6OGC), I know this is not the answer you were looking for.
Trouble is, there is no single answer, other than "it all depends."

If I had to pick one single matchbox for "this" antenna, I would choose the
Annecke 200w symmetrical koppler as being the best pick, followed by a
Johnson Viking 275w matchbox-but with the mod I show on my web site for it:
www.dj0ip.de/antenna-matchboxes/symmetrical-matchboxes/j-viking-upgrade/ 

And if I wanted to buy a new 100w matchbox for use with this antenna, I
would buy the one pictured on the page I described the antenna on the
MFJ-974B.  I've used this matchbox with this antenna in many different
locations and with many different feedline lengths and I have never failed
to be able to match all bands, albeit occasionally with a slight feedline
length adjustment.  

READING MATERIAL:
L.B. Cebik wrote an article on a similar antenna, entitled "A Vertical
Doublet for 30-10 Meters".
He used 2x 5m lengths, whereas I used 2x 6m lengths.
In the middle of the document he shows a chart of impedances by band, and
shows how much they change when you feed it with different feedline lengths.

WHAT YOU SEE in this chart is how the SWR of such an antenna is really all
over the map when trying to work a lot of bands.  That's why the choice of
matchbox is so important.

73 - Rick, DJ0IP
(Nr. Frankfurt am Main)

-----Original Message-----
From: TenTec [mailto:tentec-bounces at contesting.com] On Behalf Of Jim Allen
Sent: Monday, March 02, 2015 3:37 PM
To: k9yc at arrl.net; Discussion of Ten-Tec Equipment
Subject: Re: [TenTec] Hot Dang - Rick is a Best Link of the Week on
DX-zone.com

What "matchbox" is the right kind, for 100 watt or less use?

73 de W6OGC. Jim Allen

Sent from my iPad

> 
>> On Sun,3/1/2015 11:54 PM, Rick - DJ0IP / NJ0IP wrote:
>> In all honesty I believe this antenna is one of the best kept secrets 
>> in ham radio.
>> Takes up nearly zero space and works 80 thru 10m if you have the 
>> right kind of matchbox.
> 
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> TenTec at contesting.com
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