Antenna Mods
This tri-band radio is one of the best deals that
have come along in a long time. Plain and simple
it is a full-power 144/220/440 FM tribander with a
general coverage receiver that covers from 0.1
MHz to 1.3 GHz.

I forget what I paid for it, but it was the nominal
"street price" and to my thinking, I paid way too
little for what this thing does.

Of course, as an electronic products design
engineer, nothing is ever perfect to me - not even
the stuff I have designed. It would seem that
engineers are sometimes
content, but never
satisfied, but that's another discussion.

There were two things that I found wanting in this
tiny package; First was the SMA (type) antenna
connector. Although a good connector for various
interconnect applications, these are a bit fragile
for this kind of use. I'm sure the Kenwood
engineers were aware of that, but probably had to
compromise very valuable space for ruggedness.
The easiest way to destroy the SMA is by
stressing the antenna. There is really no support
for the thinly threaded body of the connector,
which could easily be distorted or snapped-off by  
snagging the antenna on your clothing or any
other inadvertent bump, bang or oops.
To obviate that condition, the SMA at the antenna end needs to
have a larger bearing surface. In that way, any lateral forces
placed on the antenna will be transmitted to the top of the
radio's case and not the thin walls of the connector.

My solution is shown here. It's a solid piece of rod (I used steel,
but would have preferred brass) with a 0.350 hole bored
through it and a 6 x 32 threaded set screw hole on the side.
The ID of the hole will vary, but that's what was required for the
adaptor I planned on using with other antennas. My choice was
to return to the venerable BNC-type connector.
Note that I used a long screw with a floating nut rather than an actual set screw. That will allow the use of a dangling lead which
makes an excellent counterpoise and really increases transmit range. By adding an 18" dangler (or flying lead) you effectively
create a dipole between the antenna and the dangling ground. Anyhow, here is the completed modification with a full-size 2 meter
antenna and no worries of snapping the SMA body. Note how the bushing snugs up against the raised rib on the radio's housing.
The second, and more serious shortcoming, is one which is operational; with such a small housing, there really is no room for
a decent low-band antenna. The F6 does have an internal ferrite bar antenna, but it's so small as to be very ineffective below 10
MHz. In the menu system, you can disable the bar antenna and use the SMA antenna connector for those bands. That does
work amazingly well (see photo above with coax cable and adaptors). But there is a hitch to that - and it's one I don't want to
have to deal with. Any significant amount of static charge buildup on an external antenna can (and probably will) zap the front
end of the receiver. This radio does not have bullet-proof static and impulse protection in the head end. Be very careful when
doing this kind of hookup. The antenna feeds in my shack are all DC-isolated from the outside world, so the hazard is minimal,
but you just never know when something bad will come down the pipe. Also, no matter what type of protection you may have,
arc gaps, baluns, flash-over gas tubes, diodes, bleeders, and even capacitor coupling, none of those will stop the high
frequency transient RF energy from a nearby lightning discharge. I consider "nearby" to be within 5 or more miles. Although not
foolproof, I decided to inductively couple the outside antenna with the internal bar antenna. This is a very effective and efficient
way to transfer signal energy from 10 HHz down to the bottom of the AM broadcast band. Yes, lightning transient energy can
still be coupled to the radio's sensitive RF transistors, but the odds are in your favor - the little handheld radio will be many
times safer.
I found a couple of small plastic boxes, with lids (lids not necessary) that would
accommodate the body of the radio.. I glued the boxes back-to-back. In the side
that was to be the bottom, I glued in a home brewed coil. The coil is 1/4" ID and
about 15 turns of No. 24 magnet wire. The coil is encased in heat shrink tubing to
achieve a better adhesive bond. The best location for the coil to get maximum
energy transfer is dead center under the middle of the radio's bottom. The leads
from the coil go through the back of the box to an F-type fitting. I may add a BNC
connector as well.
Here are some views
of the finished rainy
day project. Note that
the little box also has
covers. These serve
absolutely no utilitarian
purpose, but do give it
a nice finished
appearance when not
in use or stored for
The TH-F6A in operation,
tuned to .770 MHz AM
broadcast -- entertaining
me while I write this page.
I am still giving this receiver a very serious evaluation. Actually it's a very unfair evaluation since I've been tuning
around the bands for several hours and will continue more tonight after the gray line passes. The comparison
radio is a highly modified Icom IC-R71A rescued from an old government monitor/intercept station.
In due time, I'll report on the little F6's wideband receive capabilities.