Thursday, 12 July 2018

My New 80 / 40m Antenna

80 / 40m wire vertical groundplanes, now removed

For the past several years I have employed wire vertical groundplane antennas for both 80 and 40m, mounted a few feet from the ocean.

In the fall I layout the ground radials on the grass and in the spring, roll all of them up again. The main reason for going to the vertical was to eliminate problems with my 80 / 40m half-slopers when using my 1929 single tube power oscillators during the windy conditions usually encountered during the mid-winter '29 BK QSO Party.

The flexing between the tower's telescoping mast sections and the Yagi antennas that form part of the sloper's 'top hat' , cause the half-sloper's impedance or resonance to change ever so slightly. The directly-coupled oscillators respond to these minute antenna changes by constantly shifting frequency by several hundred Hertz, as their output load changes. These high winds can make the low-power oscillator difficult to copy under weak signal conditions as it suddenly jumps frequency. The verticals were immune to this 'wind effect' and performed very well ... sort of.

I've noticed, over the years, that the verticals seemed to produce a much better signal out on the eastern edges of the continent than they do in the central states and in the closer western (PNW) states. I decided to see if an inverted-V might produce a higher takeoff angle and give me a better signal in mid-continent and an 'OK' one on the east coast. The half-slopers on the other hand, always seem to produce a good signal both near and far, but could usually not be used with the one-tube power oscillators when it was windy ... which, during winter here on the coast, is a lot of the time!

Following the removal of the old wire verticals and several days of antenna building, the new 80 / 40m dual inverted-Vs were hoisted to the top of my 85' Balsam. After pruning for resonance, I have been able to run some comparisons between the 40m V and my present 40m half-sloper. The 80m half-sloper had already been decommissioned as it required the changing of a jumper to use and I was never able to resonate it properly for some reason. Oddly there was no problem with either the 40m or 160m half slopers, fed at the same point, as both resonated easily.

The new 80 / 40m dual inverted-V dipoles

The dual dipoles, both resonated to the CW portion of each band, are separated every 10' by 24" varnished and painted hardwood spreaders.

The old 40m sloper has always been a really great performing antenna (except for the power oscillator / mast flexing problem) and it was the one used for my 300mW Tuna Tin Worked All States, back in the fall of 2000. I've had good success with both the 40m version and the 160m version, with 159 countries now confirmed on topband using the diminutive 130' wire.

The 'Half-Sloper' was given a close analysis by antenna guru Jack Belrose, VE2CV, and described in QST of May, 1980. In his article, "The Half Sloper - Successful Deployment is an Enigma", he found that the radiation pattern was almost identical to that of a top-loaded monopole, fed against ground. The radiation field was largely vertically polarized and omni-directional while a lesser component, about 10db down, was horizontally polarized and coming from the sloping wire. The horizontal component had a figure-eight radiation pattern, slightly favoring the direction of the wire. He suggests that this antenna should be avoided due to the difficulty in getting it properly tuned.

My own experience with half-slopers have been mostly very positive, in spite of the tuning problems with the 80m version. To me, it seems like an ideal low band antenna for the small backyard, if you can get it to tune correctly. Assuming you already have a tower with some beams to add top-loading, hanging half slopers for the low bands takes up very little space. With omni-directional low angle radiation along with some higher angle horizontal radiation, all in a small footprint, what could be better for a simple wire antenna? Well, hopefully, my new high inverted-V!

For the past couple of weeks, as described earlier, I have been comparing the new 40m inverted-V against my 40m half-sloper, 'A-B' style, in real time, using the KiwiSDR network of online receivers. It seems that about 75% of the time, my old half-sloper, is just a few db better than the new V ... after all of the work involved, these results were both surprising and somewhat disappointing!

Perhaps I shouldn't be surprised after all, since the inverted-V configuration for a dipole is certainly less than ideal and the included angle in my antenna's normal position is only about 90 degrees. As well, the feedline attenuation losses for the new antenna are about 1db higher than those of the sloper. During the winter, I will be able to move one leg over to the far side of my neighbour's property which will open the apex up to a full 180 degrees. Recent testing at this angle has made the antennas neck and neck with one beating the other about 50% of the time, but the inverted-V is always better to the closer states.

Unfortunately, I have nothing to compare the new 80m antenna against but from listening to the signal on various SDR's around the continent and into the Caribbean, I have every confidence in it providing a much better signal than I have previously been able to muster on that band ... it should really help during the '29 BK QSO Party and the next NRR CW Party.

So what has been accomplished with the new installation? I have gained what appears to be a very good performer on 80m, as well as a 40m antenna that seems to be as good as the known good performing half-sloper, and should not cause any frequency-hopping with the one-tube power oscillators. My signal into the nearby PNW and central states has been improved. As well, now I can also avoid the annual radial deployment and retrieval needed for the wire verticals.

I'm still testing both antennas but conditions have been less than co-operative over the past few weeks. Hopefully when propagation picks up again, some of the European and far out Pacific SDRs can be used to further compare the two antennas ... there may yet be more to come!

1 comment:

Photon said...

An interesting post. The sectional tower flexing is an interesting aspect that I had not really thought of much before. I had considered bonding each section to one another, but that makes for slow retraction in a wind emergency!

I've also found half slopers to be surprisingly good, even when low. I use two wires fed off a single point, with a 12m LFA Yagi on top of a 6m mast (we are already 300 feet up, and the wind is too strong for anything taller). One gives 30m coverage with a 1.2:1 SWR. The other also gives low SWR at 40m, but has a socket for extending down to 80m. It's only just about adequate at 80m, but very effective at 60 and 40m, yielding some of the best WSPR results to be found anywhere.