Laying Down Radials

The key to effective ground-mounted vertical antenna is radials. While conventional AM broadcast practice is to use 120 1/2 wave radials, you don't need nearly that many to have an effective antenna. My shunt-fed 15m tall tower works just fine with 30 radials. None of these radials approaches 1/2 wave on 160m. Most are slightly less than 1/4 wave on 80m.

Even if they aren't very long, and don't come close to a hundred, it's a lot of wire to lay down. My vertical has about 2,000 feet of wire down today, and I'm looking to add about 1,000 more before the fall. It helps to have a system for laying them down.

The technique is very simple. The wire is laid on the ground, and the grass grows over it. After a few months, you will be hard-pressed to find the wire, or pull it up, even if you wanted to.

Start off by mowing the grass. You want this cut to be as short as you can make it without hurting the grass. Shorter grass will also make it a little easier to lay the wire.

Copper wire is expensive today, but is most compatible with the soil in most areas of the country. You don't need a heavy gauge. I started with some scrap 12 gauge cotton-covered wire I had. Insulation doesn't matter, and the wire should last longer if insulated. I would recommend 12 or 14 gauge THHN insulated solid house wiring, since it can be bought inexpensively at most home improvement stores. I would not suggest using anything smaller than 16 gauge, as the wire has got to take some abuse being on the ground. Hunt around at hamfests and offer to buy any spool of several hundred feet of wire for a buck or two.

At the base of the antenna, there must be some way to connect the radials. A few companies make expensive jigs for this purpose. I used a piece of 4 gauge copper wire, clamped to the tower legs. I put strips of stainless steel between the wire and the tower legs, to avoid dissimilar metals corrosion of the zinc plating. I then solder the radials to the wire ring using a 240 watt soldering gun. A little work with fine sandpaper makes this job go faster, as the copper wire has a thick layer of oxidation from being outside.

Once you've connected the radial to the base of the vertical, simply lay it out along the ground. As the ground is never perfectly flat, don't try to stretch the wire. Remember, we want to be able to mow over the wire. Stretching it will make it pop up in the low places along the ground.

To keep the wire close to the ground, so the grass will grow over it, I use a series of short clips. These are 3-3 1/2" pieces of 14-12 gauge wire bent into a U. You just fit them over the wire and press into the ground. You don't want to make them much more than 3 1/2", because they will bend when you try to push them in. You can generally do this by hand, but after putting in a few dozen clips, it will tend to hurt your thumb. I try to grab the clips with a pair of lineman's pliers and push them into the ground.

I generally walk along the wire heading away from the antenna, pushing the radial down with my foot and then adding a clip. If the soil is really dry, you may have trouble inserting the clips. In that case, you can wait for a good rain, or gently water the path of the radial. You'll need about one clip every 3-6 feet, so that's a few hundred clips if you are laying down 1,000 feet of wire. I built a little jig to make the bending process easier and more consistent.

For the next several days, it is good to inspect the radials to make sure kids or critters haven't pulled a wire up. I suggest not mowing for a few weeks. When you do, use the highest setting of the mower until the wire is clearly captured by the grass.

Radials definitely improve the performance of a vertical. Put down as much wire as you can, and you'll be pleased with the results.

A Few Capacitors Make All The Difference

I've been working on my little homebrew 40m receiver. When I added an internal speaker, I found that I was getting an oscillation in the audio amplifier when turning it up loud enough to hear.

A bit of sleuthing with an oscilloscope led to an unusual conclusion. My first problem was an oscillation on the voltage regulator. I used an LM7805 regulator, but raised the ground pin up with four 1N4001 diodes to give about 7.5 volts. On the voltage input pin, I could see a .5 volt peak-to-peak oscillation at about 2 MHz. There was a smaller variation on the output pin. I pulled a couple of .1 uF multi-layer ceramic caps out of the junk box and proceeded to put them across the input and output to ground. No more oscillation in the audio amplifier.

While I had the rig on the bench, I was also trying to diagnose some harshness in the presence of strong signals. What I found was that the AGC circuit was allowing some of the received audio to make it into the AGC input of the MC1350. Another .1 uF cap on the AGC input of the MC1350 helped to smooth that out.

Another issue I tackled was the BFO frequency trim only allowed a center frequency of about 850 Hz. I added a 10 pF capacitor across the adjustment trimmer and could tune down to about 700 Hz, which is much more listenable.

Only issue I haven't figured out is a third-harmonic response around 2100 Hz in the receiver passband. Probably need another capacitor somewhere else.

In any case, about an hour of work and the receiver works well with the internal speaker, and is much more listenable on the phones.

Making Noise

When calibrating the Elecraft K2 crystal filters, a noise generator becomes an invaluable tool. Using one results in clean spectrum traces. I had built an RX noise bridge some time ago from an ARRL Handbook -- the original article for that project appeared in the December 1987 issue of QST.

The noise source is a reversed-biased zener diode -- which produces noise of fairly broad bandwidth. A two transistor amplifier chain feeds the trifilar winding of the noise bridge transformer.

This circuit had the unusual addition of an oscillator to switch the noise off and on at about 1 kHz. This made for the very disconcerting effect of hearing 1 kHz tone no matter where the receiver was tuned. The noise bridge was intended to help me tune a manual antenna tuner without having to transmit, but it didn't work as expected.

The oscillator was useless for tuning the filters of the K2. I modified the circuit by removing the NE555 chip and jumpering pin 3 to pin 4, effectively supplying power to the zener all the time. (In fact, this very circuit appeared in the March 2002 issue of QST, and was also promoted by Tom, N0SS as a noise generator on his site)

However, the two transistor amplifier didn't seem to provide much output -- I got the strongest noise signal when attaching to the base of the first transistor. Perhaps this is why the tuner-tuner project didn't work so well. I suspect there's something wrong with one of the junkbox transistors I used.

My main problem was that I needed to jury-rig it with jumpers every time I wanted to use it. What I wanted was a dedicated noise generator -- which isn't much more than a zener diode noise source, followed by a broadband amplifier.

Since I didn't have much luck with the two-transistor amplifier, I thought of using something different. Mark, WA3YNO suggested using an LM703 as the amplifier. I had a better idea -- why not use a MMIC amplifier? The MMIC offers high gain, unconditional stability and an impedance near 50 ohms. I'm surprised we don't see more MMICs in amateur designs. I had a few MSA 0885's in my junkbox. Perfect.

Better engineers than I have already figured this out. If you look at the Elecraft N-gen, this is exactly what they did. So I build basically the same circuit on a piece of perfboard. The result -- S9+20 dB worth of noise. This circuit is so simple it's hard for it not to work right away. A switch is handy because the circuit draws over 12 mA from a 9 volt battery, so it won't last terribly long if you leave it on.

This noise generator works great. Make sure you don't ever transmit into it, though, as just a little bit of power will destroy the MMIC in short order. I generally use the Rx Antenna jack on the K2 (since I have the 160m module), so there's no chance of transmitting into the noise generator. Just make sure you switch the Rx Antenna off on that band, or you'll be wondering where your signals went. (Don't ask me how I know this)

Who Says Ham Radio Is Dying?

OK, I've been watching this for a while now, probably since I stumbled on the site in about 2004. Set up by Joe, AH0A, it tracks statistics on the FCC amateur license database. It has been very interesting to watch.

If you look at the table of total licenses, there are some predictable things happening: Advanced and Novice licenses have been dropping off steadily since they stopped issuing them back in April of 2000. In fact, there are so few of these, I wonder why the FCC doesn't just grandfather them into Extra and Technician class, respectively.

However, the number of Extra, General and Technician class licensees have been going up for some time. After the FCC stopped testing for CW proficiency, the number of Extra class licenses has skyrocketed. Technician and General class licenses have been on the rise, too, if you overlook all the licenses being upgraded to General and Extra.

But the exciting news is that Extra, General and Technician class licenses are at record levels, and the total number of licenses has surpassed it's previous record.

While certainly not all the licensees are active, it is good to see the increasing numbers. A healthy number or licenses should keep amateur radio alive for a long time.

Configuring Elecraft K2 Crystal Filters

The Elecraft K2 has a number of unique features, one is the variable bandpass filter which can be programmed for different bandwidths. This leads to a very flexible design for CW or RTTY, but requires a bit of configuration work. For each mode, you can select up to four crystal filter configurations (FL1-FL4), including using the KSB2 module filter (OP1). The first filter configuration (FL1) is also used when transmitting.

Typical configurations use OP1 in the FL1 position. For CW, I opted to use 1000 Hz in FL1, then with the progression of 400 Hz, 160 Hz and OP1 for FL4. The 1000 Hz setting is about the widest setting that still has smooth sides from a single peak. Much wider, and one side starts to get a pronounced "hump". This filter is good for general tuning on an active band. 400 Hz is great for crowded contest conditions. I rarely use 160 Hz, but it is useful for digging out weak ones. I centered these on 600 Hz. The OP1 filter is good for tuning a quiet band, but it is harder to zero-beat stations.

SSB and RTTY require OP1 in the FL1 position. On SSB, I used 1.8 and 1.6 kHz settings for FL2 and 3, but I rarely use them, the asymmetric filter makes for harsh listening.

RTTY necessitated a minor modification. During the A to B modifications, I added 47 and 100 pF caps across C174 and C173, respectively for the BFO stability mod. I changed these to 56 and 120 pF. However, this did not shift the BFO frequencies at all. I added 3 pF from the X3/X4 junction to ground and this lowered the lower BFO frequency 800 Hz without affecting the upper frequency. The wider BFO frequency range is necessary to accommodate the higher tones use on RTTY.

I originally picked the "low tone" frequencies of 1275 / 1445 Hz for RTTY. However, the resulting center frequency of 1360 Hz, can't be used with the KDSP2 filters. The KDSP2 filters can only be set to multiples of 100 Hz. So, I switched to frequencies of 1415 / 1585 Hz, nicely centered around 1500 Hz.

For the rest of the RTTY filters, I used 1000 Hz, 500 Hz and 300 Hz. 1000 Hz is good for general listening, 500 Hz is pretty much single-signal, and 300 Hz clips the edges of the signal passband, but is useful for trying to dig out stations with surrounding QRM.

Align the filters using the standard procedures for the CW and SSB filters, the RTTY filters are pretty much the same as the CW filters, except the center frequency is different. Instead of Spectrogram, I use CocoaModem on the Mac. If you go to the Config window, CocoaModem has a nice spectrum display. You can set the RTTY frequencies as markers in the window. You can see the picture at the top of the article shows CocoaModem displaying the 400 Hz filter spectrum, with markers at 550 and 650 Hz.

There's two things to know about setting the K2 filters. First, there's a limit to the resolution of the DAC used to tune the BFO -- it may not be possible to get the BFO of each filter exactly on the right frequency. The net result is that the frequency of a desired signal may shift slightly when moving from filter to filter. Second, it's important to match up both sidebands so they look and sound the same. Because of the VCO design, the K2 inverts signals above 20 MHz, so the filter used for LSB is used for USB above 20 MHz and vice versa. (The K2 firmware takes care of this automatically)

When setting up the filters, it's helpful to try to flip between opposite sidebands (using the CW RV button), and check that both sidebands sound about the same. Bumping the BFO setting by one tick can often be helpful in getting them to match.

Similarly, walk through the filter settings (using the XFIL button) to monitor any shift in the received signal. Moving the BFO setting one tick can help in some cases, but because of the DAC resolution it won't be completely perfect. With a little patience, it isn't hard to get the filters as close as possible.

The filters in the K2 offer excellent performance in an inexpensive radio, it just takes a bit of care to configure them.

WARC Trap Dipole

Although I've had good success using an untuned doublet on the WARC bands, I made up my mind that it would be nice to have an antenna that didn't require a tuner.

Initially, I built this antenna has a 30m / 17m dipole. The traps are made from 1.5" Schedule 40 PVC pipe, which is about 1.9" in diameter, wound with RG-59U coax. I built them in much the same manner as those I used later in the 80/40m dipole.

The 17m traps are 35 1/2" of coax wound as 4.8 turns. I trimmed these traps to 18.1 MHz. I actually built three traps, the first was a dummy form so I could figure out where I had to drill the holes to pull the coax down tight.

The 30/17m version uses 12 1/2 feet of wire in each leg, and the outer segments are 8 feet. You may have to trim the lengths to resonate the dipole in the band.

Adding 12m requires two more traps. These were made from 26 1/2" of coax wound in about 3 and 3/4 turns. Trimming the 12m traps right on frequency takes some doing, as small changes in length can shift the frequency of the trap considerably.

I inserted the 12m traps 9 feet 5 inches from the center, and then trimmed the length of the outer legs to bring resonance within the bands. I did not do a great job of measuring -- the 12m traps ended up in slightly different locations in each leg. It does not appear to affect the antenna much.

You should measure the antenna at some height, as the tuning will change as the antenna is raised. I did my initial trimming at 15 feet, and when raised to 42 feet, the resonance frequency rose over 200 kHz on 30m, and slightly less on 17m.

I built a current balun into the center mounting box for the dipole, using 10 type 43 1" ferrite beads, slipped over coax inside the box.

Since the WARC bands as so small, antenna SWR bandwidth isn't a consideration. At 42 feet, there's a little bit of a broadside pattern on 30, more so on 17m and 12m. Since I installed this antenna as an inverted V, the pattern is a bit more circular. The traps can easily handle the 700 or so watts on 17 and 12m.

The WARC bands are a lot of fun with a decent antenna. I've worked several DXpeditions with this dipole on 17m. Without sunspots, there hasn't been a whole lot of activity on 12m yet.

This sort of antenna is easy to build and install. There's no reason lack of antenna should keep you off the WARC bands.

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.