Hustler Mobile Resonator

[W4OEQ]

A local ham purchased a used 10m resonator for his 54" automobile antenna mast.  The previous owner had "modified" the coil for CB operations and we now want to restore it for 10m operations.  Does anyone know the correct number of turns in the 10m resonator coil?  We are unable to tune it for 10m in its current configuration.  The current coil has 40 turns.  Coil form is 1/2" diameter and the winding is 2" long.   73,  Tom, W4OEQ

[WN2C]

How did the previous owner "modify" the coil? Did he actually change the coil or did he cut the stinger? These resonators are fairly inexpensive. About 15 to 20 dollars depending on the band / size of resonator. Not sure how you could fix the coil if it was cut.

wn2c  Rick

[W4OEQ]

We have an assortment of stingers of various lengths and have tried several without success.  The antenna will not resonate higher than about 27.0 MHz.  (All other bands/resonators work correctly.) We assume the coil was altered because the plastic case was removed and the coil was wrapped in plastic electrical tape.  The coil presently has 40 turns of wire and there is room for about two more turns on each end.  One idea is that the previous owner removed turns.  A second idea is that the original coil was completely removed and a new winding applied.  Clearly, the soldering on each end of the coil is not original and there are some scratch/singe marks on the form.  We want to restore it to its original configuration if possible.  Thanks.  Tom, W4OEQ

[K5LXP]

One idea is that the previous owner removed turns.

Except, to resonate it at a lower frequency would mean it has more turns than stock, not fewer.

What kind of wire is it?  If it's solid bare wire you could try shorting a turn with a solder blob and re-sweeping it.  Barring that, you could start removing a turn at a time until you you net it exactly where you want it.


Mark K5LXP
Albuquerque, NM

[WX7G]

To move the RM-10 resonator to 10 meters a slightly shorter stinger is the easy solution. The stingers can be cut using a file to heavily score then using pliers snap it.

[AC4RD]

WX7G is right; usually shortening the stinger is an easy way to move the resonance.  If that doesn't work, you may have some other issue with that modified resonator.

Your pal may not *need* a resonator for 10m mobile.  I"m using the DXE 54" mast with quick disconnects and a resonator/whip tuned for each band I want to work.  For 10meters, I don't use a resonator at all--the QD holds a stinger somewhere around 18-24 inches long with a small homebrewed cap hat on the end.  That's nicely resonant on 10m with no coil--works for me!

GL!

[W4OEQ]

Thanks for the comments.  We have tried adjusting the length of the stinger but have not tried it without any stinger or with a cap hat.  We have installed a better grounding system and that helped all of the other bands.  I estimate that the present coil is 4.5 uh but I don't know what it SHOULD be.  Without knowing what the original coil was, we can only experiment with deleting turns or adding turns.....or adding a variable cap at the base of the antenna feed point.  73, Tom, W4OEQ

[W5DXP]

Can't you just short out turns until it will resonate on 10m?

[W4OEQ]

Negative.  The existing "modified" coil is 40 turns of enamel wire, close wound.  Looks like the coil form was coated with some sort of varnish/shellac before winding.  I could remove the existing coil and rewind it with fewer turns with no difficulty.  I could not add more than four turns due to limited available space.  The wire appears to be about size 18 although I have not yet measured it with my micrometer.  Tom, W4OEQ

[WB6BYU]

...I could not add more than four turns due to limited available space.

You need fewer turns.  Can you just pull the turns off one at a time and check
the resonant frequency?  I've done this before with coated coils - pulling on
the wire as I unwind it allows me to break just the bit that is holding that turn
in place.  Some acetone or fingernail polish remover might help to soften the
covering.

The charts for mobile antenna lengths and loading coils are in the ARRL Antenna
Book (and perhaps in some editions of the Handbook).  There are also some
programs and calculators available online.  You just need to know the length of
antenna below and above the resonator and the operating frequency.  (The
element diameters make a difference, but probably not enough to worry about
since you'll be adjusting the stinger anyway.)

A quick model with EZNEC, assuming a 54" bottom section and 24" stinger,
suggests that 3uH would resonate pretty close to 28.5 MHz.  As a comparison,
4uH puts me down about 26.5 MHz, so that might not quite match your
configuration, probably because there is a bit of space between the top of
the bottom section and the coil, which will increase the required inductance.

[W4OEQ]

Thanks for the insight.  Coincidentally, this morning I was reading the ARRL Mobile handbook to see if I could calculate the desired number of turns.  My ham friend came over and we re-measured everything.  In addition, we tried adding a couple of counterpoise wires thrown on the ground to see if it would change anything.  Voila!  The antenna went right to ten meters with good SWR readings.  We then tried tossing one of the two counterpoise wires into the cargo space of his Explorer and again the antenna loaded properly.  This may mean that the problem is an inadequate ground connection to his car body.  He presently is using the factory installed heavy duty trailer hitch electrical connection as a ground.  It seems to work well on all bands except for ten meters.  I would appreciate suggestions as to the best way for him to achieve an adequate ground given that the trailer hitch solution appears to have difficulties.  How about running a ground braid directly to the negative terminal on the battery?  Thanks!  73,  Tom,  W4OEQ

[W9GB]

I would appreciate suggestions as to the best way for him to achieve an adequate ground given that the trailer hitch solution appears to have difficulties.  How about running a ground braid ... ?

YES, Ground braid from the trailer hitch mount to a solid (local) attachment on the vehicle's metal frame OR Uni-body.

[W5DXP]

Negative.  The existing "modified" coil is 40 turns of enamel wire, close wound.

Actually, it's very easy to scrape the enamel off with an Exacto knife and solder the turns together.

[K5LXP]

HUSTLER  RM-10
10 MTR STNDRD RESONATOR
HRO Discount Price: $18.95

This will eliminate all doubt.  Buy another one and get on with your life.


Mark K5LXP
Albuquerque, NM

[WB6BYU]

...This may mean that the problem is an inadequate ground connection to his car body.  He presently is using the factory installed heavy duty trailer hitch electrical connection as a ground.  It seems to work well on all bands except for ten meters...

Any length from the antenna feedpoint to the effective vehicle ground (this is RF, not
DC we are talking about) adds to the effective length of the antenna.  So if your trailer
hitch mount is bolted to the frame 12" from the feedpoint, you've just made the antenna
12" longer.  That can lower the resonant frequency by up to 2 MHz.

The same thing happens on the other bands, of course, but the shift in resonant frequency
is less, and often within the adjustment range of the stinger so you don't notice it.  I've
also seen the problem with trap verticals when they are mounted on too long of a support
post above ground.

The solution is to run ground wires from the antenna mount to either side of the car
frame so the connection is as wide as possible.