Repairing (some) Mobile Antennas
Glen Roberts, KE7FD
April 2, 2017
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In this age of buying everything for the shack, I thought I would
share with those who have a defunct mobile antenna what I did to
repair an antenna that died on me during a road trip. While I am
certain there are those hams who will say the best way to repair a
dead antenna or an antenna of “lesser” design is to replace it with
something like a Tarheel or Scorpion, etc. That solution does not work
for everyone nor is that the intent behind this article. The well made
screwdriver-type mobile antennas are very fine units; I would like to
own one some day. However, for many hams, the purchase of such
antennas while justified for the sake of signal quality still
represents a high dollar expense and thus not within the grasp of
some. To those who find themselves in such circumstances and who have
purchased antennas below their aspirations, I devote this article.
I own and use the Hustler mobile antennas. They work for me and are
what I bought at Texas Towers when the Opek HVT-400 I had died early
on a trip from Arizona to Pennsylvania. Once I got home I placed the
antenna in the corner of the room until I had time to determine what
had failed. These antennas are not based on technology from the space
program (i.e., not rocket science) and can be repaired without much
fuss. I suppose similar repairs could be made to the helical type
(“hamsticks”, Outbackers) or whatever they are. The HVT-400 (or its
newest incarnation) is not an expensive antenna with a street price of
less than $60 and for 80 through 6 meters, is hard to beat. They do
suffer from a few design flaws that can be overcome if the owner is
willing to invest a short amount of time to improve upon (other than
the overall efficiency). The continuous coil (which is tapped at
strategic points for multiple amateur bands) is wound on a hollow
polymer form then covered with a sheath of heat shrink. The antenna
uses a UHF mount and is topped by a stainless steel whip. The failure
on mine ended up being the bottom most tap coming loose; the coil
could no longer be adjusted per the instructions to work different
bands. What follows is a pictorial record of what I did to repair and
improve my Opek HVT-400. Let me take a moment here to thank Shane
McDowell at The Shrink Shop
whose help made this repair better with some of his expertise.
The materials list
For this repair you will need the following:
A good quality epoxy. I have used the Lucite brand before but their
formulation is rather weak and does not cure well. Also, do not use JB
Weld since it has metallic particles that could cause problems with
this type of project (although it is an excellent product for just
about everything else I've repaired). Although I prefer the slower
curing epoxies as they generally give a stronger grip per square inch
than the fast type, a faster curing epoxy (5 minute) is a better fit
in this instance, if available.
A 200 watt soldering gun; you know, the type you dad used to use. (NO
torches here guys,)
15 inches of type 31107, 1.5 inch heat shrink tubing from the
shrinkshop.com. It comes in 4 foot lengths and other sizes may be
available, contact Shane McDowell there for current pricing.
Several inches of bare copper solid hook-up wire.
An X-acto knife or single edged razor blade or knife.
The first step in the repair is to carefully remove the existing heat
shrink. This is best done by starting an incision at one end and by
lifting the shrink as it is sliced will aid you in not nicking the
enameled wire underneath. Once done you will have this:
Carefully lift the wire that approaches a tap so a short piece of hookup wire can be threaded underneath and then wrapped around the tap. Solder this to the tap and trim the excess wire. I was able to use a soldering gun by placing the tip onto the tap opening. The heat will “flow” downwards allowing the solder job to be done before melting the plastic coil form. Pay attention that you do not distort the plastic. If you do, a following step will illustrate how to augment the support of the tap. Perform this “improvement” on all of the taps you can safely complete; I did all but the top most tap (the wire was very tight there. You may notice some slack in the different coils. Do not be tempted to remove or add windings. Let's continue to believe the engineers that designed the antenna did their homework and the coils are fine the way they are. (Figure 2 shows the hook-up wire wrapped around the tap before the solder was applied.)
Once you have improved on this design flaw of the tap only being
marginally connected to the coil, it is now time to enhance the
stability of the tap's hold onto the coil form. The bottom tap came
loose on my Opek so I started there; the order is not important. As
shown in the next two photos, you will note the “back” of the tap, how
the post is accessible at the back of the coil form (figure 3). Using
a Dremel or small file, rough up the metal to create an irregular
surface for our epoxy to grip. Also rough up the plastic around the
tap to help the epoxy bind better (figure 4).
Using the epoxy, mix only as much as can be applied in a few minutes. Since we will be applying the liquid epoxy to curved surfaces, having it stay in one place works best and this is why we chose fast setting epoxy. Apply a small amount to the backs of the taps making sure to cover the rear tap and the area around it including the coil. Do only one side at a time. After the epoxy has set up, with the antenna resting on supports so as not to disturb the curing cement, repeat the process on the front of the taps. DO NOT FILL THE TAPS! but apply the liquid epoxy to the sides or collars of the taps and the area around as before. If you have malformed any of the tap mountings when soldering, use epoxy to seat the tap properly (figure 5).
After the epoxy has cured and not tacky to the touch, prepare to apply the heat shrink tubing to the antenna.
A few words about heat shrink tubing: Because I am not an expert on
this product I turned to Shane McDowell at The Shrink Shop
to guide me to the best heat shrink product to use. Of the three
styles I was sent, their type 31107 was found to be the best
choice in my opinion. This type has a 3:1 shrink ratio which
means the 1.5 inch tubing would be snug against the coil. It has good
insulating properties. The thickness after shrinking was closest to
the original with the added benefit of an adhesive sealant that clings
to the internal components forming a weather barrier not present in
the original antenna. Look over their offerings; you may want to
coordinate the antenna color to more closely match your vehicle color.
Begin shrinking the tubing at the top (figure 8) and while rotating
the antenna, move down to the base. Also, I used a [magnetic] mount to
support the antenna during this process. Note the taps are covered by
the shrink at this stage of the repair but will be exposed later. If
you choose to use shorter lengths of tubing, start at the bottom
making sure to overlap the section below to aid in the water sealing
of the sheath.
Once you have finished applying the shrink tubing, allow it to cool completely.
With a Xacto Knife or a razor blade, carefully slice off the tops of
the tubing covering the taps removing any sealant from the throats of
the taps as you go. Multiple passes may be needed. Be neat and take
your time; once you've sliced and diced, there's no going
back. (Please note: If you loose a finger in the process, the author
of this article (including eHam) assumes no liability over your
safety. Place the severed finger on ice, apply direct pressure to your
“stump”, call 911 and wait for the paramedics to arrive.)
When you are done, the finished repair should look something like the following photos. Note the adhesive extruding from the tubing (figure 10) making a nice weather seal.