Sunday, September 22, 2019

43ft Vertical 80m Matching

For this coming winter my 80m DX antenna system will be base matching the 43 ft vertical for transmit and using the K9AY loop for receive.

Reviewing 160 and 80 Meter Matching Network for your 43-foot Vertical by Phil Salas - AD5X. Phil provided a parts list in his article which I found to be very useful.

I decided to keep it simple and just have matching for 80m. I ordered the coil stock, 4:1 unun (not sure I even need that for my setup), ceramic feedthru from MFJ.

Did a quick throw together of the components to verify that things will work as expected:

 

Rainy day hence the umbrella, screwed the coil to some wood, current choke is 7 turns of RG-8X through two mix 31 clamp-on ferrites. Not having micro alligator clips made this a bit fiddley, but I got fairly close resonance and SWR wise.

My cunning plan is to use the vertical as a support for the K9AY loop, mount the K9AY relay box inside the 8x8x4” electrical junction box containing the matching network for the vertical.

With a simple control unit in the shack, when the rig is un-keyed, power will be supplied to the relay in the junction box which will open circuit the vertical (making it non resonant), unground and connect the loop feed-points to the K9AY relay box. Figure this is safer way to do it, if the system loses power I minimize the risk of sending TX power into the K9AY relay box which will let the magic smoke out!

At the shack end I will have one coax for transmit and one for receive. Since I have an Icom IC-7300 which does not have separate receive antenna jack, I got an INRAD RX7300 receive adapter for the Icom IC-7300. This allows you to use a separate receive antenna or insert things like filters into the receive chain for example. If I loop it via an antenna switch, I can choose when to use the loop on receive.

Update 29 Sept: I got the matching network in basic form built and tuned:



K9AY relay box top right - not yet wired up with the isolating relays.

Can see the coax tap point on the front side of the inductor, the antenna end enters through the top and is tapped on the rear. In practice I got a strange SWR curve with two dips, one around 3.6 MHz, and another at 3.8 MHz. Seems I accidentally made it broad-banded somehow.

Once I got in the shack to see how things looked with a radio connected to the vertical with matching network, the noise floor on 80m was quite high, nearly S9, will need to get the receive loop up and running.

When I reconnected the ZS6BKW antenna (near by), the noise floor on 80m was lifted near three S-units - safe to say there is some interaction occurring. After disconnecting the matching network from the vertical the noise floor returned to its usual daytime level of S2.

For now I disconnected the matching network, when the relays arrive I should be-able to get this completed, and remotely switch out the matching network to eliminate the interaction between antennas.

Update 05 Oct: Relays and some mini test clips arrived. I decided I should model the vertical with the K9AY loop in its presense, it has a small effect on the vertical's resonance requiring 12 uH vs 9 uH in the model to match it, but no difference in pattern or gain.


View in MMANA-GAL with a 55ft loop (35% smaller than the standard K9AY), the smaller loop maintains performance on 80m through 40m. Larger loops with better performance on 160m lose directivity on 40m.

Some more progress on the matching network today. Relays mounted but not wired, and redid the antenna connectors with a copper plated steel strip (found in the plumbing section at Lowe's) for better grounding.




Saturday, September 21, 2019

Gain vs Wave Angle

Something I've noticed is that most don't consider is the whole picture when it comes to antenna gain. Often only peak gain at the wave angle it occurs at is quoted. Sometimes a pattern is shown with no gain figures which is useless, this is often the case with verticals.

To build a picture of what one might expect when considering low band DX antennas, I feel it's better to consider the gain at different wave angles.

To illustrate this, I created a table in a spreadsheet with several different antennas and the gain they have at different angles. LiberOffice Calc has the option to fill cell background color based on the value. The table is sorted based on the gain at 5 degrees.



I used MMANA-GAL for the modeling of these antennas. Most are 40m antennas, those that can be matched on 80m are also shown.

Wire antenna heights are 40ft/12m, the inverted Vs are 15m/50ft, the delta loops have a minimum height of 1m/3ft.

From this table I can see that one of the simplest antennas gives the best bang for buck for DX on 40, thats the half square. A small surprise is the ZS6BKW on 40m looks to have an edge over a 43ft vertical for DX! The OCFD antennas are well behind.

Disclaimer: I'm no expert at modeling antennas, I perhaps know enough to be dangerous :-)

Monday, September 9, 2019

ZS6BKW Antenna

One of the antenna projects I wanted to get finished off this summer was get the 40m OCFD up in a flat-top configuration.

Got thinking about how I'd lack an antenna that covers 75m NVIS, thought about an 80m OCFD again. At 135ft long, the current situation with tree shapes, sizes and locations leave that easier said than done, and not for a lack of trying with out resorting to chainsaws. Having said that, getting one of two 80ft Fir trees closest to the house removed is on the list, so I may be-able put up longer wire antennas in the future.

What else is there that's under 100ft, at a minimum covers 40 and 20m with out the need for an ATU, and will work on 75m?

After considering a few ideas, I took a closer look at the ZS6BKW antenna which is an optimized variant of the well known G5RV. This antenna checks the boxes on these good points:
  • Light weight - no traps, coax etc hanging.
  • Low SWR on 40 and 20m, can run ~1kW with the AL-80B.
  • Respectable gain on 20m if you have the luck or luxury of orientating the broadside to favor somewhere of interest.
  • Will work on 75m with the remote ATU I already have.
  • Also covers 17, 12, part 10 and 6m with low SWR. 17m opens occasionally during the current solar minimum, and there is the Puget Sound 10m net on Sunday nights I like to take part in.

Spent a couple weeks reading everything I could find, and playing with models - one is included with MMANA-GAL, others can be found online.

Decided it was worth trying out. Initially I was going to order the parts from The Wireman and build it my self, but found a source of these pre-made cheaper and better built for $60 (normally $90) plus shipping from Amateur Radio Supplies => ZS6BKW G5RV 80, so went that route. 

Current setup:
  • Antenna apex at 50ft, ends in inverted V configuration.
  • K9YC inspired choke balun - 7 turns of RG-8X through two mix 31 clamp on ferrites.
  • Remote ATU between the choke and coax to the shack. 

How does it work?
  • Except for 15m, low SWR on six bands from 40 to 12m, parts of 10 and 6m.
  • Remote ATU dials it in on 75, 60, and 15m, brings the total to nine bands.
  • 30m is the odd one out, ATU fails to find a match in my case.

Nine bands where only three need an ATU, with 93 feet of wire in the air is OK by me. 30m might be an issue for some, but you can't always win 'em all :-) 

Links to further information:

What's next?
  1. Get the ends up higher - the test inverted V setup I have has the ends coming down too steep and into the trees, as expected resonance / low SWR favors the low ends of the bands.
  2. Maybe after the trees are "trimmed" it's possible I could move the antenna to where the vertical is with the existing radial field. This would allow me to operate it as a T (top loaded vertical) by bridging the two legs of the ladder line together at the base, and feeding it against ground resulting in an ok 80m DX transmit antenna in place of the vertical, eliminating one antenna from my setup.

What about low band DX? Not much progress there, and with winter approaching need to sort something out. Putting up the K9AY receive loop and building an 80/40m matching system for the 43ft vertical, possibly returning it to an inverted L is the "quick fix" solution.