WARC Bands Without WARC Antennas

For those who have followed this blog for the past two years may recall that my primary objective in building my station is contests. HF DXing is my secondary priority despite comprising the bulk of my daily operating. Contests are episodic which leaves a lot of time in between!

As a consequence my effort on towers and antennas favours the HF contest bands: 160, 80, 40, 20, 15 and 10 meters. However I also need antennas for the WARC bands -- 30, 17 and 12 meters -- for general and DX operation. I want effective DX antennas on those bands without requiring effort better spent on antennas for the contest bands, with respect to performance, time and money.

As an interim measure last year to get back on 17 and 30 meters I once again put up my tried and true multi-band fan inverted vee. Originally this antenna was a key part of my Ottawa station soon after I got back into ham radio several years ago. I was very happy to find that it delivered fantastic results with QRP and 100 watts on 15 through 40 meters. Of course that was during the solar cycle maximum when a little antenna (and power) goes a long way.

When I moved to this QTH in late 2016 I temporarily hung it off the house so that I could operate until my first tower was up. I moved it to that tower in late 2017 when I moved the 80 meter inverted vee to the big tower so that I would have resonant antennas on 17 and 30 meters, and for short distance contest QSOs on 40 meters.

The multi-band inverted vee is about to be retired once more, perhaps for good this time. The reason is that I no longer need it. I am able to operate very effectively on 17 and 30 meters with other non-resonant antennas. As we'll see, even non-optimal antennas can do very well under favourable circumstances.

17 meters on the XM240

Unlike a full size dipole or yagi driven element the Cushcraft XM240 does not resonate on the 3^rd harmonic, which would be 15 meters. Coil-loaded elements is the reason. Instead the antenna has a resonance at a lower frequency, almost but not quite on 17 meters.

This has long been known and many hams with this antenna have had some success using it on 17. The additional transmission line loss due to an SWR between 2 and 3 is modest since of the 350' (110 m) of coax ~90% is LDF5 Heliax.

When I purchased the antenna I built a model in EZNEC to learn, in part, how it might do on 17. Unfortunately an accurate model isn't possible with NEC2 due to the loaded elements, As of EZNEC 5 the stepped diameter correction (SDC) does not support loaded elements but this deficiency has been partly addressed in EZNEC 6. I don't know the reliability of the new feature.

Nevertheless it is possible to gain an insight into the antenna's pattern despite the resonant frequency being incorrect.

As you can see the azimuth pattern is bidirectional, just like a dipole. There is ~1.5 db gain due to the parasitic element (a very wide spaced yagi on 17 meters), making it slightly better than a dipole. Since the equality of the two lobes is frequency dependent the azimuth pattern above is not quite what you'll get at 18.1 MHz -- I evaluated the pattern at 19 MHz to approximately compensate for NEC2 inaccuracy. In practice I find that the pattern is slightly directional in the same orientation as it is on 40 meters, so it can help to point it at the DX rather than relying on it being bidirectional.

One big advantage this antenna has is height: 46 meters up. That makes up for its quirks. I am able to work DX very well, often breaking pile ups quickly with 200 watts. The XM240 is far superior to the multi-band inverted vee with its apex at 19 meters, and it's rotatable. With the XM240 a proper yagi for 17 meters is not currently in my plan.

30 meters on the 80 meter inverted vee

The WARC bands are not harmonically related to the others. Sometimes they come close enough to be tempting. This is the case of 80 and 30 meters, where the 3^rd harmonic of 3.5 MHz is 10.5 MHz and of 3.8 MHz is 11.4 MHz. When cut for the CW segment of 80 meters the 3^rd harmonic comes within 5% of the 30 meter band.

My inverted vee is cut for mid-band so the 30 meter resonance is around 10.7 MHz. Although the modelled SWR is quite high at 10.1 MHz in practice it is no higher than 4, after accounting for loss in the long run of FSJ4 and a shorter run of LMR400; that is, it is lower in the shack due to transmission line loss. I use the rig's ATU. As is usual with a harmonically fed dipole the feed point impedance is high even at resonance.

The pattern is oddly shaped but not too problematic. As is usual with an inverted vee the polarization is primarily horizontal broadside and vertical off the ends. At its apex height of 32 meters you might expect the radiation upward should be close to nil, but this is not so.

There are two current maxima on each leg of the inverted vee -- there is a single maximum at the antenna's centre when operated on its fundamental frequency. Thereforeon 30 meters the effective height is lower and there is a big lobe pointing straight up (pattern not shown)! This lowers gain in the main lobe at 30° elevation.

Despite these drawbacks the antenna works pretty well on 30 meters. Performance depends on the direction. If this antenna does poorly on a station I can always switch to another. Which brings us to the next antenna.

30 meters on the 80 meter vertical

From on high we go down low. The 80 meter yagi array construction is ongoing and is already giving an account of itself on the bands as an efficient ground mounted vertical with its (so far) 34 radials. I will have more to say on this antenna in future. In this article I'll restrict myself to its 30 meter performance.

Like the inverted vee the vertical's 3^rd harmonic falls in the vicinity of 30 meters and has two current maxima along the monopole. The 34 radials, each 20 meters long, form a non-resonant ground plane that extends much further than on 80 meters, with respect to wavelength. In comparison to the inverted vee the SWR bandwidth is very broad, a characteristic that appears at its 3^rd harmonic. I haven't carefully measured the SWR on 30 meters but it appears to fall between 2.4 and 3. Half the 250' (80 m) transmission line is LDF4 and the rest is RG213 and LMR400. The rig's ATU must be used.

In 30 meters it compares favourably to the multi-band inverted vee and the 80 meter inverted vee. Again, direction and elevation angle are factors. The vertical is omni-directional with a higher angle lobe common to verticals operated at its harmonics.

Sometimes the vertical does better and sometimes the inverted vee does better. In a minority of cases the multi-band inverted vee equals or slightly exceeds the 80 meter antennas.

The elevation pattern assumes 5 Ω ground loss. It is more difficult to estimate the radial system efficiency when the vertical is operated on its harmonics. I didn't try since its use on 30 meters is incidental rather than a design objective.

12 meters

I left this one last since 12 meters has two strikes against it. One is that it is rarely interesting at this point of the solar cycle. Two is that I have little interest in it. There is no rational reason for the latter.

The multi-band inverted vee doesn't work on 12 meters. In fact I've never had a resonant antenna for this band. That none of my panoply of antennas comes close to resonating is therefore no great loss. The few times I do venture onto 12 meters I pick one of tri-band yagis and use the ATU.

The high SWR is not extreme and the reduced system performance does not worry me, considering my attitude towards 12 meters. A poor antenna that is up high is usually enough to get the job done when I call a DX station.

Perhaps when the solar flux rises from the dead in a few years and most of the heavy lifting is done in my antenna farm I'll take the time to put up a resonant antenna for 12 meters, even a yagi if I get serious.

Summing up

With many antennas and a little bit of height it is often possible to recruit one of them to put you on a band for which you have no antenna. Obviously this strategy is less than optimal in most cases, yet as I've discovered not optimal can still do very well indeed. It is something to consider. However if your antennas are modest and not high it is entirely likely you will not achieve superior results. This is when a resonant antenna can be well worth pursuing.

The multi-band inverted vee is still up and occasionally put to use. In fact I'm using it more right now while summertime work on the station switching and cabling make other antennas temporarily unreachable. When that is done and the next stage of antenna and tower work begins it will be in the way. Retirement is tentatively slated for August.

The antenna won't be junked. If nothing else it make a handy portable antenna should I ever want to do that, or as a loaner to a friend. This may be optimistic since it doesn't wrap up neatly due to it peculiar construction. It ends up as a tangled mess every time I roll it up, no matter how careful I am.