Keeping the simple in simplex QRP 2M FM

By

Richard Fisher, KI6SN

 

 

 

The ARRL's Field Day is a periodic reminder to me that the world of QRP need not be confined to the high-frequency bands: 1.8 to 30 MHz,

 

While other operators are pounding CW frantically on 40 and 20 Meters, I make a point to sneak away for some FM simplex work on the 2-meter band.  Terrestrial operations on 146 MHz have always been a fascination at KI6SN. It's a place where even modest antennas and low power can produce some pretty , amazing results. That was certainly the case during Field Day 2002.

 

Indeed: for portable/vacation/business trip radio activity, 2-meter FM is one of the most convenient plug-and-play QRP band and modes around.

 

With the ARRL's September VHF QSO Party on 14-16 September 2002, this is a great time to develop a VHF station and experiment with this band mode and the antennas in popular use by radio amateurs on 2 Meters.

 

For Field Day 2002 I used a three element homebrew beam made of metal rods and PVC and mounted on a PVC mast about six feet off the ground. I was running 5 watts with a Kenwood TM-261A transceiver. I was able to work dozens of FM'ers from our Field Day location 8,000 feet high in the San Gabriel Mountains north of Los Angeles.

 

Beams can be a simple and wonderful solution for 46 MHz. But, without a rotor or a permanent mast at home, I was seeking a simple solution to work simplex on a more regular and reliable basis

 

The J-pole antenna

The classic J pole has long been a solid performer on 2-meter FM. it's omni-directional, easy to tote and store and very inexpensive to build from scratch Fundamentally, the J-pole consists of parallel matching section and a carefully calculated feed point to present your HT or transceiver a 50 ohm load.

 

I was surprised, however, to learn that there are several variations of the J-pole. Versions include the single-wire radiator, the top-wire radiator and the loose-wire double-radiator, which I chose to focus upon for this month's column.

 

While reams of articles have been written about the 2-meter FM J-pole, the best treatise I've seen on the subject is by L. B. Cebik, W4RNL, of Knoxville, TN. He is one of the leading experts in antenna modeling and theory in Amateur Radio today. His four part series “Some J Poles That I Have Known" is a “must-read” on his Internet Site:

www.cebik.com.

 

Part 2, titled "The Varieties of Twin-lead J-Poles and Some Performance Standards'* gives details on building the antenna described for this column using inexpensive 300-ohm twin-lead you can find at most any hardware store or Radio Shack.

 

I was attracted to the Loose-Wire Double Radiator J-Pole (I’ll call the L-W-DR-J for short) for a couple of reasons. First, I wanted to build an antenna that could withstand being repeatedly rolled and unrolled and survive the rigors of the road. With the L-W-DR- J, as can be seen in the accompanying diagram, basically you're working with a single piece of twin-lead 56.5-inches long pretty rugged!

 


 

 


Another attraction was that the dimensions of the L-W-DR-J are in standard inch-increments that can be measured using a hardware store wooden yardstick.

 

There's a 34-inch radiator, a 22.5-inch parallel matching section and the feed-point is 2-inches above the base of the antenna.

 

The half of the twin-lead separated above the 22.5-inch matching section is the "loose wire." As W4RNL explains, this design "emerges from the desire to use the twin-lead intact for added strength. Therefore, instead of removing the wire that is parallel to the normal radiator (above the matching section), we leave it in place, cutting out only a small portion to allow one side of the matching section to be open."

 

Construction hints

While this antenna can easily be constructed in an hour or so, great care must be taken in dressing the 300 ohm twin-lead -- especially at the antenna's feed-point.

 

For the KI6SN version of the L-WDR-J, I chose to cut away a 3,4-inch long section of insulation that creates a "window” -- not unlike those seen in standard 450 ohm ladder line, in which a standard RCA phono plug is soldered to accept the coaxial feed-line.

 

In this process, an Xacto knife was used to cut away the insulation between the parallel stranded wire feed-line. Then a soldering iron was used to melt the insulation remaining directly on the wires.

 

After carefully trimming any excess bits of insulation, the builder will have a 3/4-inch length of insulation less parallel stranded wires. The rest of the insulated 300-ohm twin-lead extends above and below this ¾-inch stretch of parallel wires.

 

One of the bare antenna wires is then soldered to the center post of the RCA phono jack midway in the "window." Next, solder the opposite stranded parallel wire to the ground post of the phono jack. There's your feed point. The accompanying diagram shows how the jack is connected.

 

Next. from those solder points measure and mink the 300-ohm twin-lead two inches below the RCA phono jack. Cut the twin-lead there and trim about 1/2 inch of insulation from the parallel stranded wires. Ultimately they'll be twisted together and soldered. But let's not get ahead of ourselves.

 

From that 2-inch cut measure back up the twin-lead, crossing over the phono jack feed point, a total of 56.5 inches. Mark it there and, again, cut the twin-lead.

 

Now, identify the side of the twin-lead that is connected to the ground lug of the RCA phono jack. This is the matching section side of the antenna. Measure 22.5 inches from the bottom of the antenna and mark it. Then measure 22.75 inches from the bottom of the antenna and mark it. This is the point where you'll remove a 1/4-inch section of one of the twin-lead wires. This simultaneously creates the matching section and the "loose wire."

 

OK, now we've got an RCA phono plug dangling from a ¾-inch section of insulation-less twin-lead. That doesn't sound very sturdy, does it? A hot, glue gun can provide a solid solution. Filling the void with glue not only secures the wires and makes the RCA phono jack a solid foundation for the feed point, it's also weatherproofing for the feed point.

 

The last step is to twist and solder the twin-lead wires at the base of the 2-inch stub. Before doing that, though, it's a good idea to check for shorts. Set your digital multi-meter or VOM to “resistance" or "continuity." Touch the meter's probes to each side of the antenna at its base. If you see a short, you've got trouble. If not, you're in good shape and can now twist and solder the twinlead wires at the base of the antenna.

 

One caution: It’s extremely important to measure each element of the L-WDR-J antenna precisely. And, remember that the electrical length of the antenna begins at the tip of the twisted, soldered wires at the base of the antenna and extends 56.5 inches up the twin-lead from that point.

 

Getting on the air

I've made lots of 2 meter FM simplex contacts on 146.52 MHz using the L-WDR-J antenna suspended in a tree about 20-feet above ground at KI6SN. Signal reports from more than 65 miles away have been solid with five watts output.

 

Since my feed line has a run of about 35-feet to the TM-261A, a coaxial balun was wound using the feed line coax to prevent coupling between the J-pole and feed line. It is a simple matter of curling five turns of the feed line into a 4-inch diameter coil and securing it with electrical tape.

 

Richard Fisher, KI6SN, can be reached by sending snail mail to 1940 Wetherly Dr., Riverside, CA 92506: or by sending e-mail to: ki6sn@aol.com.

 

This article is reprinted from the September 2002 issue of Worldradio.

 

 

 

 

More on that 2-meter J-pole

By

Richard Fisher, KI6SN

 

 

 

 

 

 

 

 

 

 

 

 

 

September's Worldradio QRP column on the construction of a 2-meter J pole antenna generated a lot more than RF. The e-mail box at KI6SN was flooded with testimonials, questions and comments about it.

 

The Loose-Wire Double Radiator 300-ohm 2-Meter J Pole described in L.B. Cebik, W4RNL's online series "Some J Poles I Have Known," is a simple, inexpensive and portable antenna made from garden-variety 300-ohm twin lead and can be built in less than an hour.

 

Many who have constructed the KI6SN version featured in September say it works great.

 

Typical of the success stories was that of Jim Kephart, KD5QAG, who wrote from northeast of Oklahoma City: "As a new Ham, I could hardly wait to build the J-pole you described," he said. "it works great with my HT (handi-talkie)! I live in a rural area of Oklahoma between Oklahoma City and Tulsa. Hitting any repeater from my home is a challenge. With only the rubber duck on my Yaesu VX5R I could barely hit the closest repeater in Stillwater, a distance of 35 miles over rolling hills, and would receive marginal to poor signal reports on 5 watts. With the J-pole hanging in my front window of my home I can now access all three of Stillwater's 2-meter repeaters with 'full quieting' signal reports on 2.5 watts!

 

"I actually pulled the J-pole, by a rope, up about 20 feet into an oak tree and had good signal reports from Hams on an Oklahoma City repeater over 80 miles away. Not bad for 5 watts and an HT! I could hit Oklahoma City before only during 2-mater band openings.

 

"I'm going to use the tree approach again and try simplex operation with some amateur friends in Cushing, my closest town, and see how it works.

 

"Where I live, frequent power outages intermittent phone service and tornado weather are a fact of life. Amateur Radio has provided my family (my two sons, James II, KD5QQN and Zach KD5THZ, are now both Hams) with another way to communicate.

 

"By the way, I have requested frequency coordination for a 2-meter open repeater to be placed on top of the hospital I work at in Cushing. I'm the respiratory therapy manager. It will be supplied with emergency power access from the hospital's generator and help with local emergency communications. One enthusiastic Technician can make a difference! I have been studying and plan to take the General Class theory and CW, exams soon."

 

Jim wondered how much power this J-pole can handle. At KI6SN. I've powered up to 50 watts on occasion with no degradation in performance.

 

Meanwhile, Lionel Mordecai, K6CEQ, of Chula Vista, CA wonders “why people don't make a 6-meter or even a 10-meter J-pole using similar construction?" The source of my information for the loose wire J-pole came compliments of W4RNL, who is one of Amateur Radio's foremost authorities on antenna construction and design. If you visit his web site (www.cebik.com) and check out the four-part series "Some J-Poles I Have Known," there’s a ton of background and theoretical information that shows the J-pole will work nicely on a whole variety of frequencies –including 6 and 10 Meters.

 

Ben Bennett, N7IVM, of Olympia, WA writes: "My first 2-meter antenna was a J-pole similar to yours, but made from 450 ohm ladder line.

 

"A point which may be of interest to you was that we discovered that the VSWR was a little better when we strapped each end of the 'loose wire' back to the main radiator.

 

"I don't know whether this was related to the fact the conductor separation is greater in the 450 ohm line which I used because it was already in my possession, whereas the only 300-ohm line was old style TV downlead:"

 

Mike Greenfield, N9JIY. from Jackson, WI, writes to relay his experience with 2-meter simplex operation. "I've been monitoring and CQ'ing on 146.52 during drive times in the Milwaukee area for a couple of years now. Contacts are very tough. Only maybe 40 call signs have been collected in the whole time. Pitiful.

 

“A buddy and I tried 2-meter simplex MCW (modulated CW) and it works very FB. The MCW signal was 100 percent copyable when the FM voice signal was not even detectable. I don't know why, because MCW is not narrow band.

 

"Two-meter rigs are so cheap and plentiful, and 2-meter simplex is done so little it seems like a great opportunity to experiment and develop."

 

Mike suggests an award be developed to generate more interest in 2.meter simplex operation.

 

John La Sala, N2FWR, writes from Floral Park NY with a few concerns:

 

Q. "Regarding the RCA plug, can I substitute it for an SO 239 connector so I can feed it with a PL-259 connector? I am worried once the antenna is erected the (RCA) plug will disconnect from the antenna.”

 

A. There's nothing critical about the connector. You can use any size or style that you'd like. I wrapped the RCA phono plug/feedline connection with electrical tape at KI6SN. It has held together just fine.

 

Q. 'Do I need an antenna tuner? What is the SWR at 146.520 MHz?"

 

A. I've used this antenna at KI6SN in the 145, 146 and 147 MHz portions of 2 Meters with great success. SWR measurements haven't been taken here, but I'd welcome any reader's finding. An antenna tuner wouldn't hurt, but at KI6SN it hasn't been necessary.

 

Q. "Being a boater, if this works I will make (versions) for the marine band. Do you have specifications? It would be appreciated."

 

A. W4RNL's web site (www.cebik.com) posts formulas for calculating J poles for any frequency.

 

Bill Ross, K6MGO. writes from Marina Del Rey, CA that the construction hints in September's column say "to measure down from the RCA jack 2 inches, strip 1/2 inch of insulation that will he twisted together and soldered.

 

"As far is I can see. the shorted bottom of the antenna will now be 1 1/2 inch from the jack, not 2 inches, or am I missing something? And what does this do to the other dimensions referenced to the bottom?"

 

The goal for MGO and any other builder; is to achieve the ELECTRICAL length of 56.5 inches for the total antenna. And the stub at the base of the antenna needs to be an ELECTRICAL length of 2-inches as measured from the solder points at the RCA plug down to the precise point where the shorted twinlead wires meet at the Base.

 

But Bill makes an excellent point If, in your construction, you'd feel more comfortable adding a bit more fudge room to the stub dimension, by all means do so. For example, cut it 2.5 inches below the RCA jack connection and that'll give you an extra half-inch to play with. Just remember, when all is said and done, the stub needs to be two inches from the solder point of the twisted base wires to the point at which it's soldered to the RCA jack feed point.

 

When cutting the twinlead during construction, always keep in mind that you're trying to land on the exact ELECTRICAL lengths described in September’s column.

 

Richard Fisher, KI6SN, can be reached by sending snail mail to. 1940 Wetherly Dr. Riverside CA 92506; or by sending e-mail to: ki6sn@aol.com.

 

 This article is reprinted  from the October 2002 issue of Worldradio

 

 

 

 

GRUMMAN AMATEUR RADIO CLUB

MINUTES OF GENERAL MEETING – 10/16/02

By Pete, N2PYV

 

 

 

 

 

 

 

 

 

The meeting was called to order by Pat at 5:42 PM. All present introduced themselves.

 

TREASURER'S REPORT –

Finances continue to be in good shape.

 

REPEATER REPORT –

Gordon, KB2UB

Gordon reported that he has ordered a new Hamtronics Repeater for the Bethpage site at Plant 14. He gave the factory all the necessary parameters so that the repeater should be a plug-in when received. The cost was $1,638.00.

 

NET REPORT –

Zak, WB2PUE

The Wednesday 20-Meter Net was good this week. The Sunday 40-Meter Net was also good.

 

VE REPORT –

Bob, W2ILP

There were four applicants. Three were new hams who passed the Tech. Exam. One passed the update to General.

 

WAG REPORT –

Bob, W2FPF

No Activity

 

PROGRAM

Bob, W2ILP, gave an interesting lecture and demonstration about RTTY received and transmitted using a computer connected to a HF transceiver.