Trap Dipole for 80/40m

At my old QTH I used an 80m untuned doublet: 125 feet fed with open wire using a remote balun. I put it up my at current QTH, shortening it to about 115 feet to fit my lot.

It worked OK for a low dipole, but it was a bit troublesome. Even with a Murch UT-2000A tuner, I had trouble using it with my amplifier. It also took time to retune when switching bands.

I decided what I needed was a trap dipole. I'd had some success building a trap dipole for the WARC bands using coaxial traps. After reading the analysis by W8JI, I still wanted to use coaxial traps, but I decided to use traps resonant off the operating frequency.

You can see the result above, about half the antenna is visible, with the feedpoint on the left, and one of the traps just to the right of center.

The traps are designed to be resonant about 6.6 MHz. They are made of 76 inches of RG-59A/U, wound in 6 and 1/3 turns around a four inch long piece of 3" schedule 20 PVC. The actual frequency of resonance isn't critical, so long as both traps are identical.

The traps are wired in series, as shown below, the inner wire (black) attaches to the center conductor of the coax, and the outer wire (red) attaches to the shield of the opposite end.

The shield of the inner end is connected to the center conductor of the outer end, as diagrammed by VE6YP.

The center section is made of two 33 foot pieces of wire. In retrospect, another foot might have been better. The outer sections started at 26 feet each. I cut off 10 feet 8 inches to bring the antenna into the band, resulting in 15 feet 4 inches. If you were to duplicate this antenna, I would start with 35 feet and 20 feet, respectively, and trim from there.

Fed with RG-8x to a height of 35-42 feet, the antenna resonates at 7.22 MHz with a VSWR of 1.5:1, and 3.76 MHz with 1:1. With the typical automatic antenna tuner in modern rigs, or the pi network of older amplifiers, this antenna easily covers both 40 and 80m. Being so low, this antenna doesn't have much of a pattern on 80m, and only a gentle broadside pattern on 40m.

I've been pretty happy with the dipole. It works well on 80m early in the evening when the propagation is short. Not bad for an antenna that's less than 100 feet long.

I hope to add a couple more traps to add 30 and 20m this summer. I'll let you know how that works out.

My Elecraft K2 Wish List

I've owned my K2, SN #2548, for several years now. It has been my primary radio since the fall of 2002, when I added the KPA100 module to it. I really enjoyed building it, and I'd like to build another. A couple of years ago, when the K3 first came out, Wayne Burdick at Elecraft collected wish lists of K2 features. Given the runaway success of the K3, I have my doubts if they will ever get back to enhancing the K2, but in the spirit of hopefulness, here's my list:

Firmware-only Suggestions:

• RTTY Mark Display Frequency - Just as CW displays the carrier frequency of the received CW signal, the RTTY mode ought to display the frequency of the Mark signal. A menu option would be needed to set the frequency of the Mark signal. (This would essentially subtract or add the Mark frequency from the LSB / USB carrier frequency, respectively)
• VOX Toggle - I'd rather toggle VOX on and off, and have the VOX delay time set in a menu. Having to cycle through all the VOX delay times to turn VOX on and off is a real pain.
• Tuning Rates should follow mode - 10 Hz tuning is fine for CW and RTTY, but too slow for SSB. 50 Hz tuning is fine for SSB. The rate button could then be used to switch to 1 kHz step and back to 10 or 50 Hz. It would save a lot of button presses trying to get the right tuning rate. You could also display all the digits of the frequency regardless of tuning step (in which case SSB would alternate the bottom digit between 0 and 5)
• RIT / XIT Display - momentarily display the RIT or XIT offset on the main display when it is changed (much like for power or keyer speed).
• Unified Filter - Rather than having to separately select XFIL and AFIL settings, I'd rather have three of four filter presets that allow me to combine an XFIL and AFIL setting for each mode. One button (say XFIL) would then cycle through the presets. That way, you're always selecting an optimized filter solution, rather than selecting it manually, which is made more difficult because there is no filter display indicator.
• IF Shift - This very handy feature could be implemented with a firmware change. Biggest problem is how to control it. I find the XIT to be useless, so I would vote to replace XIT with IF Shift. An alternate would be to use the Keyer Speed knob in SSB or RTTY modes.
• CW Tuning Direction - an option to flip sidebands on 15m and above, so that the tuning direction stays the same. (This won't work on RTTY or SSB, of course)
• CW Memory Command - have a computer command to write to the CW memories. That way you can program them from the computer and use them from the front panel.
• INP mode for CW PTT - This mode would allow PTT on the dot line and CW keying on the dash line - great for contesting work. (this would disable auto-detect of hand keying)
• Variable Rate Turning - normal tuning is one step at a time, but if you turn the knob faster, the tuning rate increases for as long as you tune quickly.
• Auto-Tuner Out -- Allow one-touch selection of Tuner AUTO/CAL. This would allow one to easily switch the tuner on and off.

Hardware Improvements:

• Improved KSB2 module -- starting with a filter with sharper skirts (better than 1.5 shape factor), 6-10 dB more mic gain available, more positive VOX (using separate VOX amplifier), anti-VOX circuitry.
• Improved KNB2 module -- allow selection of longer blanking times, more thresholds, better blanking action.
• PFx Module -- a tiny little module that has four (or so) buttons. Hooks into the aux line and offers access to eight functions of the K2 (four on tap, four on hold). Obviates the need for FPLY, opens the K2 up for easier access to things like switching filter presets, noise reduction modes, RIT clear. Lots of simpler and cheaper than the KRC2, because it is just the buttons. (This might also be useful for the K3)
• Remote Tuning Knob -- borrow the idea from Ten-Tec. Perhaps part of the PFx module.
• Opto-encoder for RIT/XIT knob -- Replace the potentiometer with a optical encoder. That way, we wouldn't have to manually re-center it.

I intend to keep using my K2 for several years. It's my hope that Elecraft will be able to deliver some of these enhancements.

Cushcraft A3S/A743

As mentioned before, I've been using a Cushcraft A3S for several years now. I purchased it back in 1989, had it up for about six years, in the basement for five, loaned out for a couple of years to W1YM, and then back up since 2001 at my QTH.

There's many things to admire about the A3S. It's quite rugged, having survived several freak Georgia ice storms, but it only weighs 25 lbs even with the 40m option. While it's one of the last tribanders designed without computer modelling, it has reasonable performance on a small boom of 14 feet. It's been a popular product both in home installtions and even for Field Day. I have one ham buddy who used to use one on a pneumatic mast mounted to his conversion van.

Here are some of the things that I have learned about the A3S and A743:

• Tape the driven element insulator

A layer of electrical tape protects the driven element insulator from the Sun. There's not much of it exposed, but the sunlight will slowly break it down over time. Covered with tape, it will last forever. Use a sharp knife to open the U-bolt holes.

• Tape the trap end caps

The plastic trap end caps won't last forever. After about 5-8 years in the Sun, they will split. Cushcraft also thinks quite a lot of them -- replacing all 24 end caps cost me almost $50 in 2000. Try to preserve them by placing a single layer of tape around the circumference of the cap where it goes over the trap tube. The flat ends don't tend to split, since they aren't under any tension.

• Keep the trap holes down

While it is in the instruction manual, it's pretty easy to have an element section rotate and have one or more holes pointing anywhere but down. Leave them that way, and the traps will fill up with water quickly. Get the screw clamps on tight enough so the element sections don't rotate.

• Mount traps the correct way

The traps are designed so that the end connected to the cover tube is closest to the boom. Make sure all of them are this way. If you are not sure which way they are, you can carefully slide an end cap off to check. Mark the trap tube with an arrow using a Sharpie so you can remember. If you put the trap on backwards, it may be hard to diagnose.

• Use a balun

Cushcraft recommends making a balun by taking six loops of coax and taping them together. WA2SRQ wrote a posting to TowerTalk back in 1996 that showed that such a balun is much more effective when the turns are in a single layer and do not overlap or bunch. Six turns of coax on a 4 inch PVC form should work nicely. Ferrite beads a foot and a half long works well, too. I found a set of 10 of ferrite beads large enough to fit over RG-8 and wired them underneath the boom.

• Twist the element support rope

With the A743 option, the driven element droops far too much, so Cushcraft includes a non-conductive support rope and support mast. The instructions show the support rope should have a twist right at the top of the support mast. The twist prevents the rope from sliding back and forth through the plastic grommet as the driven element moves in the wind. It will eventually break. Don't ask me how I know -- just remember to include the twist.

• Element spacings

The A3S offers three choices of spacings along the elements, CW, MID and Phone. I took a tip from K7LXC. With the Cushcraft 40-2CD,

Steve recommends going half-way between the CW and MID settings.

The last time I had my A3S up, I used the Phone settings, and the SWR was very high at the bottom of the bands. So, I used values that were midway between CW and MID.

This results in a good compromise in covering the entire band. Here's the SWR curves through about 130 feet of RG-213.

Based on these graphs, I could probably shorten up the spacing between the 20m and 15m traps, and the 10m traps and the boom.

Regardless of the values you use, double check the measurements before putting the antenna up. When I first put up the A3S, the 10m director spacing was about six inches too short. You couldn' t tell from the SWR curves, but the antenna likely did not work as it should.

There may be better tribanders you can buy, but the Cushcraft A3S is a great performer. The A743 option adds 40m. At my QTH, this is my best 40m antenna, partly because it is mounted higher than anything else. Being able to rotate the pattern is often helpful in crowded contest conditions.

Phonetics

There's been a few letters in QST lately about the proper use of phonetics. The latest QST (the one from the future -- July 2009) has an op-ed piece about phonetics.

The original letter that set this recent discussion off was a couple of months ago. The writer complained that he heard a station signing "London Radio" and thought he'd be talking to England, but was disappointed when the station was merely in the US.

That was probably me.

The writer insisted that everyone should just use the standard phonetics, and never anything else. While well intended, I'm afraid that the ICAO phonetics, while they work very well for aviation communications, don't always cut it.

"Romeo" -- this is just not a great word. It works OK most of the time, but not everyone in the world knows Shakespeare. "Radio" is a word that all hams know. It's the one phonetic that will get through when nothing else will.

"Lima" -- this is just a weak word. Watch your wattmeter when you say it -- it doesn't have that much punch. Using it, I get all kinds of guys who heard everything but the "Lima". They think it is "Charlie" or "Kilo" or "Echo" -- anything but "Lima". "London" works more often than not. About the only place that "Lima" is recognized well is in Central and South America.

"Alpha" -- try saying it twice. It comes out kinda funny. A lot of guys hear "Alpha Delta", but I also get "Delta Alpha" and "Papa Alpha" responses -- particularly from stations in europe who may be more familiar with those prefixes. At that point, the longer "America America" seems to work -- it communicates the letter as well as the country of origin.

In short, I've had this callsign for 25 years now. I've had all sorts of experience with what gets through and what doesn't. The standard ICAO phonetics don't cut it for my callsign. Pardon me if I use something a little unusual -- but it works. And isn't that what ham radio is about -- communicating?

30th Year as an Extra

I was writing a note in reply to a ham who was upset that a certain DXpedition had posted operating frequencies that were all in the Extra and Advanced-class portions of the bands. He was worried that as a General-class licensee, he wouldn't have the opportunity to work the expedition.

Of course, he had nothing to worry about. These expeditions regularly listen for callers in the General-class portions of the bands. I did point out to him that between now and the time of the expedition, he had plenty of time to upgrade.

Then, it hit me. It has been 29 years since I took the Amateur Extra-Class exam.

Exams these days are pretty easy, compared to conditions years ago. Back in 1980, the FCC still administered the exams. For Extra class, code was 20 wpm, and you had to take a total of three written tests -- one for General, Advanced and then Extra. Plus, the exams were only given at FCC offices at major cities -- so, if you lived far away, you faced quite a road trip.

It was the end of my Freshman year at Georgia Tech. My family had come down to visit my grandparents, and take be back to West Virginia after finals. My brother, then KA8DTD, figured he might be able to schedule an upgrade exam before the week was out. Sure enough, amateur exams were available on Friday. Ben had a Conditional class license, and I was a General, having been tested at a hamfest just a year before. So, he scheduled both of us to take the Extra exam. Friday, June 13, 1980.

Neither of us was terribly worried about the code test, although I hadn't been down to the club station in weeks. I did borrow his Extra-Class Study Guide for that week, and managed to go over it in the midst of studying for and taking final exams.

Friday morning came, and our father dropped us off at the FCC examination building. We filled out paperwork and sat for the code test. I sweated it a little, but passed. Ben passed with no problems.

Then came the Advanced written examination. I figured it would be easy, since I studied for my Extra. Wrongo -- the old Advanced test was the largest of the bunch, with 50 questions, all of them tough. Only my private pilot's written exam was harder. 

We both passed the Advanced written, and so far it had been worth the trip -- we were both guaranteed an upgrade. After the Advanced, I was convinced I was going to flunk the Extra -- but the test turned out to be a lot easier -- only 40 questions, and most of them I studied for.

After sitting through three exams over several hours, I got the news. I had passed! I was an Amateur Extra class. Ben didn't fare so well -- he had missed the Extra by one question. And that was probably because I "borrowed" his study guide all week.

So, this begins my 30th year as an Amateur Extra-Class. Hard to believe it has been so long ago.

Carlisle Tires...BOOM!

Camping is fun. While tent camping has it's attractions, it's not nearly as convenient as taking an RV. With an RV, you can pack many of the conveniences of modern civilization. There are several choices with an RV. Years ago, a buddy and I invested in a camping trailer. The biggest advantage of the trailer is that once you get to the campsite, you don' t have to break camp to go somewhere, as you might do with a Class C or Motorhome.

While we sold that trailer a few years later, my family found we liked trailer camping. In 2002, we bought a new Travelstar 21SSO. It's easy to tow, light, and has plenty of room. It came with five ST175/80R13 tires made by Carlisle.

Trailer tires take a lot of abuse. They are small, and are asked to carry a lot of weight. These tires are designed to run at 50 psi cold, and the manufacturer recommends the trailer be towed with the tires inflated to that level. Even a slight under-inflation can cause tire failures.

Imagine my surprise a couple of years later when I lost two trailer tires on one trip. We were coming back from the forgotten coast of Florida, and one blew out in south Georgia. I didn't think anything of it -- I just put the spare on and continued driving home. This turned into a disaster near Byron, GA. I was watching my remaining tires in the rear-view mirror and saw one go flat. By the time I could turn off the highway 2 miles later -- the tire was completely shredded.

After losing two tires in one trip, I didn't want to repeat that experience -- so I did the most rational thing -- I called the company who sold me the trailer. I was trying to find out how I might prevent failures in the future. The dealership couldn't offer any information, other than the number of the tire manufacturer. When I called the manufacturer, before I could even ask my question, the person I was talking to told me they wouldn't give me any money. Huh?

A year later, we were headed for Florida again -- and just short of Tifton, GA, we had a tire blow out so forcefully that it broke my gray water drain valve. We immediately sought a tire store that had the right size tires. Not only did I replace the blowout, but also another tire that was showing an unusual bulge. At this point, none of the tires I was running on were Carlisle tires. I had the shop move the only remaining Carlisle tire to the spare.

Funny, I haven't lost a tire since. Last year was the final straw. I was prepping the trailer for a trip, and went to check the pressure in the spare. Now only did it not have any pressure, it would not pump up! Taking the tire cover off quickly showed why. This tire had a large crack across the tread -- which is why it didn' t hold air.

So, in six years, all five Carlisle tires failed. And the one tire, the spare, which had less than 150 total miles on it, had cracked across the tread. Today, I have three different brands of tires on my RV, and none of them have failed since they were put to use.

A couple of years ago, I asked a friend of mine who has been trailer camping for over 20 years with an A-liner how he dealt with blown trailer tires. He said in all the years he's been camping, he'd never had a tire go flat for any reason.

A co-worker of mine also bought a TravelStar 21SSO about two months after I did. He also had Carlisle tires. After three blowouts of his own, he has replaced all of them with Goodyear tires.

What conclusion can you draw from this? In my experience, Carlisle tires are defective. They clearly cannot meet the basic requirements for tires of this size. If you have any of these tires, I would strongly recommend you replace them with another brand. If you do this now, before they fail, you can shop for bargains -- rather than having to buy a tire where ever you can find one on the road.

I've had good luck with three Denman tires, even the Chinese-made SuperTrailer tires have outlasted the Carlisles.

Good tires are important on an RV. Nothing spoils a good camping trip like not being able to get there (or get home).

Shunt-Feeding a Short Tower on 80 and 160m

I've operated 80m since I was a Novice. With a little room in the yard, it's not hard to put up a basic 80m dipole. Getting an 80m dipole up high enough for it to be effective is not so easy.

160m is more of a challenge. A simple dipole doesn't fit in your typical yard. A quarter-wave vertical requires quite a support structure. What we need is a effective compromise antenna.

I think it was my involvement in the NAQP that got me determined to get on 160m. I had managed to make a few contacts loading whatever antennas I had and trying to call the loudest stations on the band. After I had my tower up, K9AY encouraged me to get on the ARRL 160m contest with my 80m dipole. He also suggested that I try to put up an inverted-L. I but one up for 160m with two 100 foot elevated radials. I found the 80m dipole was more effective. 

W8JI once wrote that his full-sized 160m dipoles at 300 feet were only more effective than his verticals a few percent of the time, and that was only during very unusual conditions at sunrise. The way I interpreted this was that vertical antennas can be very effective on 160m, perhaps also on 80m. What I needed was a way to put up an effective vertical antenna for 160m and 80m.

K9AY originally suggested I try to shunt feed my tower. His advice was to run a wire all the way to the top of the tower sections, then measure the impedance at the bottom, then design a matching network to match the resulting impedance. So, that's exactly what I did.

You can see the upper shunt wire mount in the photograph to the right. A piece of 1"x1" angle aluminum U-bolted to the tower provides the support for a piece of 3/4"x3/4" angle aluminum that separates two 12 gauge insulated wires. The wires are about 10 inches apart and join again another piece of 3/4"x3/4" angle aluminum fastened to a NEMA matching box at the bottom of the tower.

The shunt design has been through several iterations at my QTH. First was a single wire that was matched for 80m using an omega match. This worked ok, but I started with just seven 50-foot radials. When I added eight more, the shunt worked much more effectively.

At this point, I felt that all those radials might work on 160m, so I added an L-network that I could switch remotely with a relay. This worked until I added the A743 40m add-on kit to my Cushcraft A3S. The omega match no longer would load on 80m. 

The Mark III match used a common tapped inductor for 160 and 80m, and a relay that switched in variable capacitors for each band. I had a lot of trouble with arcing. The Mark IV is what you see below. It uses a separate L-network for each band, switched in or out with a DPDT relay. Everything is contained within a 6x6x6 NEMA box shown below, which is mounted on a 1"x1" angle aluminum U-bolted to the tower.

160m is on the left. The coil uses two stacked T200-2 cores with 42 turns of wire tapped every two turns. Capacitor is a dual-section which I measured at 15-325 pF per section. The small silver-mica capacitor has been replaced with a 4 kV 160 pF disc ceramic part. The result is about 180-800 pF of capacitance, of which I'm using about 750 pF. 

80m is on the right. Coil is 38 turns of wire tapped every two turns. Capacitor is a 13-250 pF unit that was the plate tuning capacitor on a Heathkit HW-40.

The photograph shows enameled wire, but I've since re-wound the 160m coil with insulated wire. Biggest problem is that the heating and cooling cycles of being outdoors cause the turns to rub against each other. This eventually results in arcing between the turns when RF is applied.

The matching networks aren't designed for a lot of power. The 80m network can manage about 200W, and the 160m network only 100w. Higher power networks would require beefier networks. Except for the NEMA box and angle aluminum, this was all stuff I had in the junkbox. 

Does it work? Well, I have WAS confirmed on 160m CW, with three states left to go on 80m. For DXCC, 21 confirmations on 160, and 34 confirmations on 80. Yes, this antenna works! The key to making this antenna work well are the radials. I have twenty-five 50-foot radials, plus four 100-foot radials. Or, at least, I did. Some recent septic-tank work cut several of the radials.

I plan to lay down more this summer. I put my radials straight on the ground, held close with small loops of copper wire pushed into the ground. This keeps the wire down until the grass can grow over it. Once the grass covers them, I have trouble finding the radials.

If you have a short tower, consider shunt-feeding it to get on the low bands. It's worked well for me.