Building and Using the 80 Meters Loaded Half-Sloper

The prospect of operating QRP on 80 meters is not particularly enticing to me, especially considering my primary interests of DX and contests, even if I stick with CW. Although I need to do it for at least contest operating I had been putting off building and installing the half sloper design I described in an article 6 week ago.

The procrastination ended this past weekend. Two hours in the workshop and then a rushed hour of outdoor work (rain was imminent) was all it took. The morning temperature was a cool 5° C, which is surprisingly comfortable for doing tower work. That is, in calm air.

The antenna works even though it'll never win a prize for exceptional performance. You can refer back to the September article (linked to above) for design details. This article is about construction and my initial experience with the antenna.

Measurements

The modelled length of the half-sloper wire is approximately 17.8 meters, including a 1.25 meters long section going outward horizontally from the tower (due to NEC constraints). With a loading coil of 17.5 μH, medium conductivity ground, the tower and yagi the result is modelled to resonate at 3.550 MHz with an SWR of about 2 (low radiation resistance). However the model is certain to be inaccurate to some degree due to a variety of factors, including: ground, environment, acute angle between wire and tower, and exact contribution of tower width, guys and yagi.

As initially constructed the actual measurements are as follows:

• Top of wire is 14 meters up, or about 10 cm below the top of the tower.
• Coax terminates at 14 meters height, with 14 AWG insulated wires from an SO-239. Wire length to the tower connection is ~30 cm, and ~60 cm to the sloping wire.
• The sloping wire is 12 AWG insulated wire. The top section length is 11.5 meters and the bottom section length is 6.0 meters, with the loading coil between them.

Feed point

Mechanical design of the feed point is shown in the picture below. The wires from the SO-239 are green and the sloping wire (hanging vertically) is black. The set of tower X-braces is the very top of the tower, just below the top bearing plate. You can see the right edge of the egg insulator that terminates the top of the sloper wire. It is closely roped to the tower leg behind the mast.

Yes, it's ugly. After considering constructing a more aesthetically pleasing feed point I decided to keep it simple, yet functional and effective. Even if it stays for a year it is an experimental antenna not worthy of extra effort to make it look pretty.

Sealing putty (used Coax-Seal) protects wire lugs and the SO-239 backside from water incursion. An ordinary electrician's twist connector connects the sloper wire. A screw lug on a strip of sheet aluminum connects to a tower brace with a hose clamp. All hardware is stainless steel.

The longer wire to the sloper can be trimmed as necessary, or the sloper wire can be trimmed at the bottom end. In its present lengthy format the wire to the sloper wire is going all over the place, including pressing up against the mast. This will be cleaned up before the snow flies, as will the final waterproofing of the coax connector.

The potential between all conductors is low throughout the mess of wires so there is no risk of flash-over or erratic tuning. There is no common mode choke on the transmission line since the outer shield will in any case couple to the full length of the tower. A 1:1 current balun to choke common mode current can be used where the coax exits the tower if induced noise or RF in the shack are present. For the present I will go without one.

Loading coil

Per the design, the loading coil is ~65% down the wire, with 11.5 meters above it and 6.0 meters below it as first cut (see above). I used a 17.5 μH coil I picked up in a flea market, which I bought since it looked to be about the right value, and the length-to-diameter ratio is consistent with a low ESR (equivalent series resistance). The inductance is calculated since my inductance meter only resolves to 10 μH.

As shown in the picture I used a dog bone insulator to take the strain of the wire tension; there is no tension on the coil itself. Screw loops and (soldered) connections to the coil are made from solid12 AWG copper, to keep the insulator centred within the coil. The loops permit the use of #12 stainless steel hardware, rather than soldering, to join the wires and coil.

The result is ugly but effective. It's also light enough that the wire sags very little even with low tension.

Anchor

After coming down the tower I walked the antenna towards Europe (northeast) until the bottom end lifted off the ground. With a brick and nylon rope I temporarily anchored the antenna at the edge of my lawn.

The wire end is ~40 cm above the ground. Based on the wire length the calculated interior angle between wire and tower is 37°. This is 8° less than the 45° maximum I will allow to ensure no negative interactions with the yagi.

The temporary anchor will be replaced by a pole that will be anchored to the retaining wall. This will increase the wire angle a degree or two and alter tuning, but is absolutely required for safety.

Safety!

Speaking of safety there is a risk to life with this antenna, and I don't just mean tripping over the wire and falling into the neighbour's rose bushes. The end of the sloper wire and, very importantly, the tower bottom carry high RF voltages when transmitting with more than QRP. With a kilowatt there can be a risk of severe RF burns or even electrocution.

If the tower is grounded the risk at that point is partially mitigated, depending on the quality (impedance) of the ground connection. However you should always assume that the tower is a serious safety hazard at 100 watts or more. Be prudent and take measures to limit the danger.

The end of the sloper wire has no mitigating factors, so the RF potential can be very high. Do not leave any conductor exposed at the termination. Let me tell you a true story about how serious the danger can be.

Back around 1986 I had an 80 meters half sloper (full size) on my 20 meters high tower, also oriented towards Europe. Since I was not so smart in my younger years I tied the bottom of the sloper wire with string to a cinder block anchor positioned adjacent to a steel shed. That placement just happened to give the best match. To improve the match I later added a foot of insulated wire to the end and just let it rest across the string and block.

That night I got on 80, switched on the Collins 30S1 and did some DXing on CW. Minutes later the SWR shot up. Since the 4CX1500B is a delicate tube I reduced drive and did some testing. At low power it worked better but would misbehave at more than a few hundred watts.

The next morning I went outside to investigate. The end of the extra length of wire had drifted close to the painted steel of the shed. There must have been some fireworks since a 10 cm square section of the shed had the paint burned off and soot radiated outward. The wire was a carbonized mess. I had inadvertently created an RF arc welder.

Tuning and match

As originally cut the 2:1 SWR bandwidth of the antenna is ~180 kHz and dips to 1.1 at 3.450 MHz. This is 100 kHz lower than planned. This is either good planning or good luck since this is equivalent to 1 meter of wire length, and I had added an extra 0.8 meters for tuning (trimming).

The minimum SWR and SWR bandwidth are better than the EZNEC model. This does not surprise me. It has been my experience that half sloper of this type, with the wire end so low, tend to match well to 50 Ω systems.

I suspect the reason for the differences is loss in the ground and adjacent buildings that is not encompassed by the model. That is, an additional series loss resistance raises the feed point impedance close to 50 Ω, and that also lowers antenna Q. The additional loss isn't good but the match is, perhaps, some compensation. There are limits to what can be achieved with a simple low-band antenna on a suburban lot.

On the other hand the high Q and narrow bandwidth could be due in part to pushing NEC2 to its limits, as briefly discussed in the design article. This could mean that environmental losses might not be too bad. Unfortunately I don't have a way to properly test either hypothesis, except by on-the-air results.

The antenna can be trimmed at the bottom or by trimming the mess of wires at the feed point. In the former case it can help to raise the antenna off the ground. I opted for a bit of both: taking half off the top and the rest off the more-accessible bottom. If you like, a coarse and fine adjustment, respectively. However it does not do to make all the changes at once (including a permanent, safe-to-walk-under anchor) since any change affects the geometry. The change in wire angle and distance from ground can affect resonance and match more than a change in wire length. My approach is to get the anchor properly positioned and only then adjust the wire as needed.

Final tuning was not done as of the publishing of this article. It will be routine. For operation higher in the band in CQ WW SSB this weekend I will use a tuner. Tuner and transmission line loss will likely be no worse than -1 db. Alternatively, a short section of wire at the bottom of the antenna can be manually switched in and out to choose between CW and SSB segments of 80.

Interactions

I spent some time in previous articles discussing the possibility of interactions among antennas, a serious concern with so many wires antennas and yagi in close proximity. The interaction model I built with EZNEC helps identify problem areas, and how to avoid them. Now that the 80 meters half sloper is installed you can see the mess of wires around the tower in the adjacent, annotated photograph.

The actual distance between wires is not obvious since there is a loss of 3D perspective in the photo. The north upper guy wire is about 2 meters from the end of the 40 meters element of the multi-band inverted vee, and it is a similar distance from there to the half sloper wire. Looks bad, doesn't it?

In actuality there is no measurable interaction between antennas, with respect to SWR or (so far as I can tell) pattern. This is in accord with the modelling experiments I did. Provided that the antennas are no more than mildly resonant with respect to others and the crossing angle is far off the parallel the mutual coupling is small.

This is good enough for my purposes but not necessarily for others. Coupling to the outer surface of coaxial cables can bring RF and local noise sources into the shack and receivers. If you use more than one radio at the same time, whether in a contest or for weak-signal DXing, these types of interactions should be considered and solved with common mode chokes placed at strategic points.

On the air

Just my luck that by the evening of the day I installed the antenna a long period of geomagnetic stormy weather set in. US stations were heard but little in the way of DX. I listened to a few weak German stations during the WAG contest, and the large US contest stations trying to work them. There was no hope for me. It takes a DX opening to properly test the antenna so I waited several days until the ionospheric absorption relented enough for DXing to become possible.

In the interim I measured the SWR and tested antenna interactions. I did not tune the antenna until after making some QSOs. The SWR of 1.5 at the bottom end of the CW segment was good enough to begin operating.

On Tuesday I made my first QSO with the antenna: W1AW/8 in WV. I called a weak LZ station who was having a sunrise enhancement but he got only part of my call. But that was very promising, telling me that the antenna has possibilities even under poor conditions. Remember than I'm only running 10 watts.

My objective for CQ WW SSB this weekend is more modest: to make some short-haul QSOs to bump up my score. I will continue trying my luck at CW DX as 80 meters conditions permit.