A 30 and 40 Meter KX2 Adapter for the Grabil CF-2200 Carbon Fiber (or any other cheap) Whip
Executive Summary (don't ya feel important now?)
I've been very pleased with the Grabil CF2200 carbon fiber whip for handheld use on the KX2.
The whip is lighter and stiffer than the metal whips I'd been using, and the KX2 readily tuned the CF2200 alone from 10 through 20 meters.
But that left me without 30 and 40 meters. As good as the KX2 tuner is, it refused to match the Grabil on either band.
This post describes how I made a prototype 30 and 40 meter adapter that can be left on the CF2200 so that all bands 10 - 40 meters can be tuned by the KX2 tuner.
No reason you can't use this with any cheap extendable whip to build a poor boy's AX-4, but I find that the CF2200 provides field advantages, so I focused on that.
The Background
I've operated /PM with simple 7 to 8 foot whips on the KX2 for several years now.
Other blog posts here describe how I've built a simple bracket to mount most any kind of a whip on the KX2 without stressing its BNC, and I won't belabor that point. You can find bracket details here: A Nearly Universal Whip Adapter for the KX2.
The pic below shows the whip doing its stuff in a 30 MPH wind on the shore in Oban, Scotland last month. Love how it performs in the wind compared to heavier, bendy metal extension whips. Really missed 30 meters, which has always been a good band for me in the late afternoon overlooking salt water.
The Design
Because the KX2 tuner is so good, the CF2200 should only need a coil (or two) that gets the whip within the range of the tuner.
This gets you to the natty issues of calculating how much coil you need, and building a solid mechanical assembly to hold the form and attach the form to the KX2 mount and the Grabil.
The Form
As much greater geniuses before me have done, I turned to half inch ID PVC pipe for a form. It's easy to find and work.
Half inch PVC end caps work well with 3/8-24 bolts and nuts, which in turn mate with my whip adapter and the Grabil whip.
The attaching parts of the form are made by drilling 3/8 inch holes in the ends of two end caps, inserting the bolts into the end caps so that the threads extend outwardly, and securing the bolts with 3/8-24 nuts.
As seen below, bolts with 3/4 inch of thread are about ideal. Note that there are two styles of bolts, a standard bolt, and an Allen head bolt.
The round Allen head bolt fits easily within the cap, while the hex head is a tight fit and will need to be pounded in, pulled through by tightening the nut, or reaming the inner surface of the end cap. The Allen head bolt is much easier, and you know why I know.
EASY 😎😎😎😎
Another PRO TIP, you can make your life much easier by drilling a 7/16 inch hole rather than a 3/8 inch hole. The bolt passes through easily and the larger hole gives you a bit of wiggle room to center the bolt when tightening, just in case your hole is a bit off-center.
Those same nuts will be used to secure the ends of the wire coils that will be wound on 5 1/2" piece of the PVC pipe. This is a finished end cap.
The Coils
My junk box had a roll of 20ish gauge enameled copper wire. One inch of PVC coil form holds 22 turns of that wire when close wound.
Coil inductance is always a thorny question, but because the coil only needed to get the system within the range of the KX2 tuner, I didn't sweat it much.
I first built a one inch test coil so that I could measure its inductance. Then I used an on-line Coil Inductance Calculator to calculate the inductance of the1 inch, 22 turn coil having a diameter of 0.665 inches. The calculated and measured inductance of the test coil were within 3 % of each other, so the model proved a good basis for moving ahead.
If you're wondering about how I arrived at the coil diameter, I used a Harbor Freight digital micrometer to measure the width of the PVC form and the width of the wound coil. I then averaged those two numbers to get an estimated coil diameter. Exact measurements were necessary to get the test coil and the model to converge.
Next step - determine the coil inductance required at resonance. The same site provides a Coil-Shortened Vertical Antenna Calculator. I used this to determine the base-loading required to resonate a whip having the length of the Grabil CF2200 plus the length of the base loading coil.
Fiddling with the numbers a bit, it looked like a whip the length of the Grabil and the loading coil assembly would resonate on 30 meters with about 8.9 uH of inductance.
In a fortunate coincidence, an additional 8.9 uH inductor would resonate the system on 40 meters.
A little more calculating with the Coil Inductance Calculator gave the result that my coils needed to be two inches long, thus 44 turns, each. A jumper wire from the base of the lower loading coil to a tap at the top of the upper coil would short both coils on the higher bands, and a tap between the coils would allow me to short one of the coils when I wanted to operate 30 meters. The jumper would not be used on 40 meters. And Bob's your uncle.
Of course that's not exactly what happened. Earlier I tried to be too clever by half - literally. I wound a single coil of 12 uH inductance hoping that the KX2 would tune that system on both 30 and 40 meters. But it would not reliably tune both bands.
So that's why I ended up with two coils instead of one. Nothing ventured, nothing gained, I guess, and it's what forced me to build a two coil system that would resonate on each band.
Building the Adapter
At this point, building the adapter was pretty easy. I:
1) cut a 5 1/2" piece of the 1/2 inch PVC for the coil form;
2) marked the centerline of the form;
3) drilled two sets of holes on either side of the centerline for threading the top and bottom of each coil. Each set of holes was 2 inches apart (two inches of 22 tpi winding). I left half an inch between the two coils to have room for connecting the jumper and to keep the two coils from coupling much;
4) wound the coils;
5) twisted the inside ends of the coils together in the middle, soldering them to make a connection point for the coil jumper;
5) fitted the end caps;
6) looped the outside coil winding wires under the nuts on each end cap;
7) tightened the end cap nuts; and
8) soldered the alligator clip jumper wire to the bottom of the lower coil.
What I didn't do was glue the end caps to the coil form. I did not do this because nothing ever works the first time for me.
Except, of course, this time. I haven't bothered to take the adapter apart to glue the end caps to the form because it's working just fine as is. But it's probably a good idea to glue the caps on, least they decide to come off in the field.
The Results
It worked well enough!
Testing on 30 meters with the 16 foot counterpoise wire I use routinely yielded a match with SWR below 1.5 to 1.
On 40, to be honest, I infrequently get an outlier in the SWR 3-4 range. But typically the system tunes at an SWR of 1.7 to 1 or so, and the outliers typically come in below 2:1 on the next tuning cycle.
Eventually I will rebuild the adapter with something better than the alligator clip jumper. But the electrons don't care, so I'm not in a hurry.
Of course, YMMV, particularly with different counterpoise configurations.
But the point of having two separate coils at or very near resonance for 30 and 40 meters is so that the tuner has some room to roam. My guess is that other counterpoise configurations will be easily tuned by the KX2. But testing continues.
Always looking for something for nothing, I tested the adapter on 60 meters, but alas, no joy at all. It's very apparent that as frequency decreases, you need to be close to resonance to be sure the tuner will match the system.
The Fine Print
As always, I write about my experiences, but these are not recommendations, just documentation of those experiences.
It is always up to you to decide what is safe for you, the people, property and pets around you, and your equipment.
This is especially true when you are waving an eight foot whip around on a handheld radio. Be careful, especially around any powered lines and other people.
And of course, have fun!
Please send any ideas for improvements to ka9p@aol.com and I'll happily credit you and post them here.
Cheers, Scott ka9p/zf2sc
Comments