[ARC5] Tuning up BC-230 and ARC-5

[mstangelo at comcast.net]

I use a Matchbox Jr. tuner to match a 100 foot inverted V feed with ladderline.

I use this as an all band antenna 80 through 10 meter antenna.

It does a good job on most bands, including the WARC bands but I could not get a decent SWR on 10 and 40 meters.

I wanted a quick way to increase the matching range without modifying the tuner. I fabricated L brackets to mount 5-way binding posts on the the balanced feed studs. I made 1 inch diameter coils from heavy gauge enameled wire I had an hand. I found I could get a good match on 10 with a single turn while the 40 band required 30 turns. I then mounted the coils on a bracket with banana plugs so that I could easily swap coils to change bands.

You need a symmetrical antenna with feedline perpendicular to the antenna to get a balanced feed. I was worried about common mode currents on the feedline so I but some Fair Rite 31 toroids around the feedline where it enters the basement.

I have another Matchbox Jr, which was hacked. I am intend to modify this by switching the number of turns on the input link coil, similar to the "Annicke Mods".

Mike N2MS 


----- Original Message -----
From: Dennis Monticelli <dennis.monticelli at gmail.com>
To: Kenneth G. Gordon <kgordon2006 at frontier.com>
Cc: ARC-5 List <arc5 at mailman.qth.net>, AKLDGUY . <neilb0627 at gmail.com>
Sent: Thu, 31 Mar 2016 14:32:37 -0000 (UTC)
Subject: Re: [ARC5] Tuning up BC-230 and ARC-5

All my antennas are balanced and so are the feedlines and so is the tuner.
It's important to maintain balance throughout the system if one wishes to
keep the feedline from either radiating or picking up signal (including
noise).

The most overlooked part of this fully balanced system is the tuner.  Many
of the so-called balanced input tuners are not well balanced at all.  QST a
while back ran a collection of tuners (both modern and vintage) through
their paces with various loads and on all bands.  They also measured the
degree of balance.  The hands down winner for balance was the Johnson
Matchbox (both KW and Jr).  I use a Matchbox Jr.   The ferrite balun based
balanced input tuners did not fare well and were particularly bad on the
upper bands.   Of course you could roll your own balanced tuner and do a
good job if you pay attention to details.  Remember even if you do a great
job installing a balanced feedline should the "terminations" of the
feedline (i.e. antenna and tuner) not also be well balanced, then unequal
currents are going to flow on that feedline and it will become part of the
antenna whether you like it or not.

As an aside, many hams turn away from a tuner like the Matchbox because
they cannot get a perfect SWR on all bands.  Even after playing with
antenna length or feedline length there always seems to be one band or two
that doesn't want to play along.  So they turn to a more modern tuner that
has a wider tuning range.....that is also lossy and not well balanced. Well
there is a better solution.  You can use the unbalanced ATU in your rig to
clean up residual SWR or you could use a good quality unbalanced external
tuner in series with the Matchbox.  On the bands where the SWR is already
good via the Matchbox, just run the other tuner on bypass.  On the problem
bands as long as the SWR is "dipping" with the Matchbox, there is a
resonance going on within the Matchbox so it's doing its job acting as a
wonderful balun.  It just can't transform the impedance all the way to 50
ohms.

Dennis AE6C