Finishing the 160 Meter Gamma Match

When the time care to redeploy the 160 meter antenna for the winter season, there were a few improvements. These are mechanical changes that make the system better able to survive the elements. Since there were no electrical changes there is no difference in performance. However, I did include a provision for future expansion.

The new features of the shunt-fed 160 meter tower:

• Weatherproof enclosure
• Additional support for the 60' long gamma rod (2-wire cage)
• Improved RF high voltage insulation and isolation
• Easier adjustment of the feed point impedance
• Expansion room for additional antennas
  

Gone are the margarine tub at the feed point and the unstable bottom termination of the wire cage gamma rod. These are holdovers from my aborted attempt last year to improve performance.  The new system is built to last.

The one thing that's still there is the flimsy radial attachment band made from aluminum flashing. It'll be replaced in time. It works well enough and it is easy to attach radials. What it lacks is strength. When deer (or errant hams) trip on a radial, the aluminum band easily bends. While it hasn't been seriously damaged yet, it may be a matter of time.

The new capacitor enclosure is staked on its supports. The strut to the cage gamma rod telescopes and is insulated from the tower and nearby cables by the pivoting ABS pipe surrounding it. Recall that the potential across the gamma capacitor at 1000 watts is close to 2700 volts. The isolation is critical for safety and performance.

This is a side view of the strut. There are hose clamps on either side of the ABS pipe to hold it in position after adjustment. The rear clamp also holds the wire from the capacitor. There are two hose clamps for the telescoping section. A length of aluminum angle provides a far better bottom termination than the temporary system it replaces. The gamma cage wires are held 40 cm apart from top to bottom.

The gamma rod wire was also replaced. Previously I used soldered together scraps of #12 stranded THHN since I didn't want to waste good wire on what was at the time an experiment with a cage gamma rod. The new wires are unbroken lengths of #14 solid black THHN taken from a decommissioned wire antenna.

The ABS enclosure is pretty tough and has room for a large transmitting variable capacitor. It's black for UV resistance. The sealed cover has to be removed to adjust the capacitor. I didn't want to pierce the enclosure for an adjustment that typically only needs to be done once per season.

The aluminum angles that comprise the mounting brackets were hammered into the ground with the enclosure attached. That worked out better than trying to make the holes line up after pounding them in first. I broke up the sod where they penetrate the ground so that they could be pressed down with minimal force.

Aluminum strips hold the fixed transmitting capacitors, and there is room for one more if needed. The variable sections are in series to prevent arcing; combined this way it's good for up to 4000 volts. The usable range of the gamma capacitor is 150 to 175 pf. A small capacitance (high reactance) is needed to cancel the large inductive reactance presented by the gamma match.

If you look closely at the left side of the enclosure you'll see two openings for coax connectors that are currently taped over. Those are for future antennas that will share the transmission line via relays. I have lots of Heliax but burying it is a bother. I'll use these for non-contest bands such as 30 meters so that sharing doesn't cause a conflict during contests. 

A new trench and Heliax runs will only be prepared when I have more contest band antennas on this tower -- it currently only has the 20 and 15 meter stacks. The 160 meter vertical can be used at the same time as the stacks.

I hoped that a cleaner installation would help broaden the SWR bandwidth but it didn't work out. I expected better based on ON4UN's Low-Band DXing. It's a problem because, not only is the voltage very high, the SWR bandwidth remains narrow. It is no better than 70 kHz for an SWR below 2. 

Amplifier tuning must be done for frequency changes of more than 10 or 15 kHz. That's annoying during a contest. For the ARRL 160 meter contest I placed the frequency of minimum SWR a little higher than shown. I lowered it now that I'm back to mainly DXing on top band, CW and FT8 at 1840 kHz.

I don't know why the cage gamma rod doesn't increase the SWR bandwidth. From all the research I've done it ought to do so. With an electrical height of ⅜λ the impedance is very sensitive to the position of the tower tap point. Even so it ought to be better. Going from a single wire to a cage made no difference. I'm stumped. More modelling investigation is in my future.

The one thing I did achieve was to reduce the gamma rod spacing. It used to be between 2 and 2.5 meters (8') from the tower. That's a lot. With the new system the separation is now about 1.6 meters (5'). Getting to that magic 50 Ω value was easier. That's a relief. I used to dismantle it over the summer since it interferes with antenna work. I may be able to leave it in place year round. Only the radials will need to be rolled up for the hay season.

In windy weather the SWR oscillated due to the motion of the 60' long gamma rod wires. Tension on the wires helps but is insufficient. During one wind storm the wires twisted together. I made two improvements to eliminate the problem. 

At the 30' level I installed a support for the wires. Both the long strut and the cross piece are PVC pipe. The wires fit into slots and are held there with wire ties. A few inches of aluminum angle are bolted to the cross piece and held to the strut with a u-bolt. The tower clamps allow for full 2-dimension adjustment. (Apologies for the lack of a close up but I'm not climbing to take a photo!)

The strut to the gamma rod pivots at the tower to allow ease of tension adjustment. I first tried this with the rock on a wood plank that leaned on the end of the strut. That created some havoc with the impedance. As already discussed, the bottom of the gamma rod has a very high impedance so it is very sensitive to foreign materials that are poor insulators or that can absorb water. 

The thin mason line doesn't affect the impedance. A small rock tensions the wire and anchors it to the ground. In 70 kph winds we had following the work, the impedance variation was negligible and nothing shifted position.

That's it for this year. I need to do more investigation to discover the reasons for the high inductive reactance that is the cause of the poor SWR bandwidth. I already determined that the tower tap point is critical, but it is difficult to tame the reactance while also putting the resistance within reach of 50 Ω. It's annoying even though amp tuning can deal with it. 

Despite my complaints, the antenna still works great. That's worth a lot.