6 METER, 5/8 WAVE MOBILE WHIP

   I had just finished putting the finishing touches on my six-meter fixed-station installation, and I was trying to think of what I could do to improve the dismal performance of my six-meter mobile installation.  My problem is that I have a full 1/4-wave whip on top of the aluminum shell on my pickup, which really lacks performance when trying to work skip.  Unfortunately, no mobile skip contacts yet.  It has a great match and all.  It just has a lousy radiation pattern or something.

   I have had this long, stainless steel tank whip kicking around for around twenty-two years, and had yet to figure out what to do with it.  This problem finally gave me the idea to use it as a 5/8-wave antenna for six.  It looked long enough (it was longer than a quarter-wave CB whip which some use as a half-wave on six).  The trick would be how to match up the whip to coax since a 5/8 wave does not have a 50-ohm feed point impedance. 

SOME QUICK ANTENNA STUFF

   A regular quarter wave whip, as mentioned earlier, is a reasonably good radiator.  It has a couple of shortcomings though.  And it all has to do with simple engineering.  First, a quarter-wave element has a high take-off angle when mounted on a flat metal surface.  This is great for working high-level repeaters and such, but lousy for skip or fringe-range work (unless you live in a deep canyon or such).  Second, (and this is something that most hams, who have not dived into some antenna theory, miss) is that a quarter-wave radiator mounted on a flat, or almost flat surface, does not have a 50-ohm feed point impedance at resonance (more like 12 to 30 ohms depending on the installation).  So, after a ham prunes his quarter-wave whip for a flat SWR, you have actually de-tuned the antenna from true resonance to get the good SWR.  This is efficiency lost, but, in this case, a compromise is usually made in the interest of making the transmitter happy, and for simplicity.

   I thought about trying to make a half-wave antenna, but trying to match up such an antenna would require more math, inductors, and capacitors than I wanted to deal with.  And I would go through all of that for only about 2 Dbi of gain.

  So, I decided to go all out for the 5/8-wave radiator.  Not only would it give me a full 3 Dbd of gain at least, but it would also really flatten my radiation pattern towards the horizon instead of trying to talk to the cumulus clouds overhead.  And I figured I knew how I was going to match it to the coax.

HOW CAN THIS WORK?

   Now let's get something straight.  I am not an RF engineer, or anything of the sort.  All it took for this non-math whiz to make this work was some copycatting and some crude mechanical engineering.  It also helps if you know how to solder properly as well!

   The idea for making this antenna work was, quite simply, the study of a dismantled 5/8 wave, 2-meter antenna.  No, not for using it as a quarter-wave six-meter antenna, which many people do.  I noticed that the base assembly had a coil in it.  About four turns or so.  This was the source of my idea.  The coil was in series between the coax and the whip.  There was no shunt-feeding or base-loading going on here.  No series or parallel capacitors involved (In other words, no complicated matching network requiring a buncha math to figure it all out).  The coil simply acts as an impedance-matching device.  Although when you use a 2-meter Larsen as a 1/4 wave on six meters, you are using it as a base-loaded antenna.  The coil simply changes its function according to the R.F. applied to it.  Neat, huh!  It is not what the designers at Larsen had intended, but, who's complaining?

   So, now I had the basis for my idea.  Now, I needed to copy it and hope the same principle worked on six meters as well.

BUILDING AND MOUNTING THE ANTENNA

Well, there was a small amount of math involved.  I had to figure out the length of a 5/8 wave at 54 MHz.  That turned out to be 135 inches (add a few inches for 50 MHz).  I measured the whip that I had, and it was about 140 inches long.

Here's a hint for you H.F. operators: A 5/8-wave 6-meter whip is also roughly equivalent to a 1/4-wave at 15 meters.  A great antenna for mobile users of feed point-type automatic antenna tuners.

   The mount I was going to use was the only thing I had available at the time, a Shakespear right-angle mount that is meant to be installed on a boat, and used for a fiberglass H.F. antenna.  I bought it at the swap meet for a buck, figuring, like most of us junk collectors, I would find a use for it (someday).

  Since the mount and quick-disconnect I would use will add several inches to the electrical length, I pruned the whip down to 133 inches.  This would leave it a little long, but that was o.k.

    At first, I installed the mount at the front of the shell on my truck.  Then, I made a coil.  Four turns of #10 solid copper wire (removed from an old chunk of ordinary household Romex cable), about 3 inches in diameter, about three turns to the inch spacing.  I soldered a lug onto one lead, and left the other end unterminated, and then installed it on the mount, where the coax center would normally be connected.  I soldered the center conductor of my coax to the unterminated end of the coil and grounded the shield to the aluminum shell.

TESTING AND TUNING

   When I checked the SWR, it showed a three to one at 50 MHz, and over that (way over) at 54 MHz.  This result was actually better than I had expected.  I had expected much worse for something I was just tinkering around with.  I was actually expecting the SWR needle to pin! Then I would have thrown my hands up and went back to the drawing board, so to speak.

   I had to figure out why the match was off.  The only reason I could come up with was that I had too much conductor on the coil, in other words it is electrically too long.  The antenna was the right length.  The coil was not.  The hard part of this was now to begin.

   I unsoldered the coax center conductor and clipped an inch and a half off of the coil, and reconnected the coax.  When I checked the match, it had gone down a bit.

Aha!  I thought to myself. I'm on the right track.

   It took several rounds of this activity, but in the end it was all worth it.  When I was done, I had the 1:1 match that I thought I would never attain.  My theory had proven out to be good.  But, what would it do on the air?  I was about to be pleasantly surprised.

   I drove up to a church located on a hill about four blocks from my QTH.  I was able to contact Joe (N6SIX), and the test was on.  Joe is located about five miles (and around a couple of hills) from where I was parked.  Unfortunately, he was operating a Ranger RCI-5054 so the signal reading would be vague to say the least.  While operating five watts of output power on 52.525 F.M., I asked for a reading while I was on my quarter-wave whip (my regular antenna).  His response was that I was giving him about 1/3 scale on the Ranger.

   Next, I connected the 5/8-wave whip, and had him check my signal.  He replied that I was now full-scale on the Ranger radio.

"Yessss!",  I thought after Joe gave me the reading.  It proved that the antenna really did have at least some gain over my quarter wave.  I was feeling pretty good about the whole thing.  And I was patting myself on the back pretty hard.  After all, it was quite a bit of work.  And Joe's report made the whole effort worthwhile.

   Isn't this some of what ham radio is supposed to be about?

   The antenna created some problems.  The obvious one was its height.  When you mount a whip of approximately 12 feet on a 6 1/2- foot tall platform (my camper shell), then go driving down the street, you're bound to hit things, particularly when driving around on surface streets.  The second problem was the weight of the whip.  At the base stud, the tank whip I was using was almost three times the diameter of a CB whip you can get from, say, Radio Shack.  The result was when the whip hit something, it did not give or flex well and it would transfer a severe physical shock to the mount.  In fact, I almost lost the antenna when the mounting hardware started to rip out of the aircraft aluminum my old shell is skinned with.  So I moved the mount to the back of the shell where I could screw into the steel frame of the shell.  Then, I almost lost the whip again when the Derlin mount itself broke off after the whip smacked a very low tree branch on the main drag at 40 MPH.

So now I had a broken mount to tell me it was time for some kind of improvements.  I got very lucky when I found another Derlin Shakespear mount at the swap meet ($1).  The next problem to cure was the weight of the whip, which meant I could not use the heavy tank whip anymore.  So I went to Radio Shack and bought a steel quarter-wave CB whip.  I bought it because it is thinner and lighter than the tank whip I had been using.  I ground off the aluminum ball at the tip, then I extended the whip by clamping another thin whip segment to the top.  First, I measured the total length that I was short and rummaged around until I found a whip segment that would give me the additional length I would need without too much overlap.  Using what electricians call a krunch clamp (see picture), I clamped the two together.  The complete assembly weighs about half what the tank whip did.

   The next task was to make another coil like the one that was wrecked when the original mount broke.  I went through the effort to make a coil, only to get a sudden bright idea after I had made the coil, but (thank goodness) before I had gone through the trouble to install and tune it.

   The one big disadvantage of the coil was that, once it was tuned, you were limited to the segment of the band that the antenna and coil was set for.  The antenna will have about a 2 MHz (2:1) useable bandwidth.  So, depending on your operating preferences, you could tune it for the FM part of the band, or the SSB sub-band.  If, for example, you set the antenna for SSB, you would have to live with a 2:1 or higher SWR on the FM parts of the band.  This bad of a match will cause most late-model transmitter's ALC circuits to start to cut back or restrict their output power (self-preservation mode).

A BRIGHT IDEA

   My solution to this problem (and remember, I did not know if it would work) was a roller inductor.  The idea was that I could set the inductor for one segment of the band (say, FM), then change it with a small adjustment of the coil, if I wanted to work the other segment of the band (SSB).

   I went to the swap meet to look for a roller inductor.  I was not looking for any particular brand, model, inductance range, or anything like that.  What I would eventually buy would be based totally on what I thought would appear to do the job.  More specifically, I was looking for one that had an inductor diameter that matched, as close as possible, the diameter of the coils that I had been making by hand.  When I was done, I had walked out of the place with an E.F. Johnson (part no. 229-202) roller inductor along with a few other impulse-buy items (you all know how that goes!).  The part was in clean condition with nothing broken, sprung, tarnished, no hardware missing, etc.  It even had the insulating ceramic and metal spring flex coupling (a rare item in itself) installed on the shaft for use with a shaft extension (I replaced the coupling with a plain black knob).  For ten bucks, the whole thing was a nice find!

   I installed the inductor right next to the antenna mount to keep the jumper lead running from the inductor to the antenna mount as short as possible (see photos).  The coax center lead was soldered to the tap (roller) terminal and also the near end terminal of the coil and a short (about two inch) insulated wire connected the antenna mount to the other inductor terminal.  I ran a length of coax from my base radio (it has a mic with a locking TX switch) out to my truck.  An SWR meter went in line at the vehicle.

   I spun the inductor until it was tapped at about 1/3 of its total range to the antenna.  I set the radio for 52.525 and set the power to the five-watt setting.  Then (after proper I.D. was issued) I locked the radio in transmit.  The SWR, as I had assumed it would be, was in the red.  I started to slowly rotate the inductor so that the tap was moving towards the antenna connection (reducing inductance).  To my immense relief, the SWR indication started to drop.  But would it drop enough?  Could this entire assembly introduce some kind of weird reactance that I had not planned on, and prevent the match from going flat?  That was my big worry.

   When the roller tap reached a point at about three turns from the antenna lead, the match dropped flat!  To say that I was elated was an understatement.  I was so happy that my crude theory had worked out so well.  But, there was still another important test to make.  Would it also adjust to the SSB sub-band?  I unkeyed the radio and changed the VFO to 50.150.  I keyed the radio and ran back out to the truck.  The SWR now read 2.2:1, which is what I figured that it would read after the frequency change.  I rolled the inductor a little more than half a turn away from the antenna and the match dropped to 1.1:1.  I was now 2 for 2!

   But I was not done yet.  I wanted to try one more thing. I spun the knob until the inductor was completely shorted out of the circuit, changed the VFO to 21.2 MHz (15 meters), and checked the match.  It was almost flat.  I then changed the VFO to 18.12 MHz (17 meters) and, again, put just enough power into the antenna system so that the SWR meter would work (about 4 watts).  I slowly rotated the inductor until I saw the match drop flat.  This proved that the inductor would also act as a base-loading system on some of the lower H.F. bands.  I was able to obtain an almost flat match (about a 1.2:1) in the middle of 20 meters as well.  And at this point, I had not even used half of the inductor!  When I tried to load up the antenna on 40 meters though, it was too much to hope for.  The match would only drop to 3:1 at resonance (about 3/4 of the inductor in the circuit), a condition I speculated was caused by the lack of a large enough ground plane area for the antenna to work against.  To make the whip match up at this frequency, one would need to install a variable capacitor into the system and create a tank circuit.  This was more work than I wanted to go into at this point.

   The only problem that I really cannot cure at this point is the height of the whip.  It is simply something I will have to live with.  It is tall enough that it will tickle the bottom of a few bridges, tap a few low-hung phone and CATV wires (power service drops are usually maintained at a height that precludes hitting them), and, of course, the inevitable tree branch.  These problems do not occur when I am belting down the freeway at speed because the whip leans back enough in the wind to prevent it from hitting anything.

   If you install this project on, say, a standard car, or the bed of a small pickup, you will not have most of the problems associated with the height of the whip (except for your garage or carport).  A Hummer or an Escalade?  That is another matter!

A HINT FOR AUTO-TUNER USERS

   If you are a ham who uses a CB whip on a vehicle with a feedpoint-type antenna tuner on the lower HF bands, you can extend the whip just like I did and help your whip radiate more effectively.  If, for example, you are a heavy user of 15, 17, or 20 meters, a CB whip with a three foot or so extension clamped or welded onto the end will help it work much better as the whip will be that much closer to resonance, especially at 17 meters.  An auto-tuner will also allow a standard CB whip to be loaded up as a half-wave on six meters if your tuner will operate on that band.  Good results have been reported in this configuration as the antenna will have a small amount of gain over a 6M quarter-wave whip.

     Good luck!

     73 de Stacy