40m wire antennas compared

Before going on holiday I managed to get some life into my old C-Pole using a new home made choke. I re-tuned the antenna (with some challenges) but did not get a chance to use it. 

In the past I did use it quite a bit and was pleased with the results but as always - this does not tell you a lot. HF conditions vary too quickly and dramatically to base any evaluation on single antenna experiences. You really need simultaneous A/B testing.

A gap in my schedule allowed me to go out and do some testing of different wire antennas for 40m. I ended up testing a C-Pole versus an end fed halve wave more or less vertical and the same C-Pole versus an inverted V dipole.

Test setup

I used two identical WSPRLite beacons that transmit with 200mW on the WSPR frequency. 

• The C-Pole was set up so that the feedpoint was at approx 2m above ground (that would be your typical setup with a 12m pole, like the Spiderbeam I have - with the top of the antenna at 11m). 
• The EFHW was attached to my 18m pole and therefore almost vertical. The last 3-4m I set up sloping so that the feedpoint was at approx 1.5m above ground.
• The Inverted V was set up so that the feedpoint/apex was about 13m high. 

The test period was the end of the afternoon. Not the best time for 40m and certainly not the best time to test DX performance. However it was the time I had available. 

C-Pole meets EFHW vertical

The first run was the C Pole vs the EFHW vertical. I let the beacons run for about an hour. After that time I had:

Antenna	Transmissions	Spotters	Spots
C-Pole	15	60	328
EFHW	15	60	392

On first glance the EFHW vertical seems to fare better. It has been heard more often. However both antennas reached the same amount of spotters. So nothing too dramatic here. There is not a lot of difference in the stations that received the signals - so most of the spotters returned data for both antennas.

Now looking at the SNR reported by the spotters there is more to say about the difference. I averaged the reports per spotter to decrease the amount of data points. The graph below shows the signal strength reported (SNR in dB on the Y axis) at the spotters' distance. It looks like the EFHW (in orange) performs better.

I then calculated the average difference in signal strength per spotter of the EFHW over the C-Pole - so only for spotters that returned reports for both antennas. The graph shows the difference the EFHW scored in dB - above the line means the EFHW beat the C-Pole. 

This clearly shows the EFHW beats the C-Pole on almost all distances and sometimes by a fair margin. The outlier is S51RS at 950km. That is the only spotter favoring the C-Pole significantly.

So if you have enough height available the EFHW is the one to choose out of the two at least for the ranges tested today. This is what I expected from the theoretical analysis I did some time ago.

An interesting followup would be to see how the EFHW would perform with the same top height (sloping from the 12m pole or tree branch). With a slingshot one can easily get a vertical wire up 20m, so I would normally aim for that height with this antenna.

C Pole meets inverted V dipole

Now how about the inverted V? This one adds another element to the game as it is horizontally polarised (as opposed to the vertical EFHW and the C-Pole).

I chose a height around 13m (not fully extending the 18m pole). In retrospect I might as well have gone 2 meters lower to compare the antenna if set up on the same 12m pole as I used for the C Pole antenna. Here we have a slight advantage for the inverted V.

Running the beacons for just under an hour I had:

Antenna	Transmissions	Spotters	Spots
C-Pole	10	45	202
Inverted V	11	46	219

The inverted V got one extra shot so it is hard to tell from this first glance which one performed better. 

Looking at the average report per spotter shows that the Inverted V (in green) seems to beat the C-Pole on most occasions. 

Moving one step further the indication is confirmed. The Inverted V wins (above the line) almost everywhere. The biggest outlier again is S51RS.

Conclusions

I did not test the inverted V against the EFHW vertical directly. However looking at the differences per test, using the C-Pole as "a reference antenna", there does not seem to be a lot of performance difference within EU. I would have expected the inverted V to do slightly better than the vertical on the shorter distances (with its higher radiation angle) but that might be only noticeable on even shorter distances.

So, this time I learned that within EU the C-Pole loses against an inverted V at more or less the some top height and loses against a vertical EFHW with the feedpoint at more or less the same height. 

One question remains - as the antenna's have different radiation patterns - would any of these clearly beat the others on multi hop DX? The simulations I did previously would suggest that the C-Pole would have an advantage over the inverted V because of more low angle radiation.

If it turns out the C-Pole does not "deliver on promise" there, then I cannot see a lot of situations where I would choose it over one of the other designs. Only if the footprint needs to be minimal and height is restricted (*).

This last question requires a new test around gray line time - when I can't use my favourite test ground as it is only accessible in daylight. Something to put on the "to do list".

*) Another one: would the C-Pole beat a shortened 40m EFHW of equal height - so approx. 12m? I have such a wire with spool that I used in the past for my 10-20-40m EFHW.

C-pole tuning part 3

This weekend I went out to test my C-pole with a new 1:1 balun. As I blogged before I ran into trouble with this antenna and the common mode choke I constructed in W2DU style.

To rule out any balun influences I had decided to tune the C-pole last week using my VNA without any balun present. My idea at the time was that this would give me the most clean picture of the antenna and would rule out any failure of the balun ruining the tuning process. Smart move? No. (but I learned something).

While working on the "naked" antenna I was amazed how much off it was. To me at the time it seemed strange (I tuned it before didn't I?) but it did explain why it failed in operation. So I continued re-tuning the antenna - moving the feedpoint a considerable bit along the one leg of the folded dipole (basically the C-pole is a dipole with the ends folded in a square shape towards each other).

After 1,5-2h of pushing the antenna up and down again I had a perfect match in the 40m band. One happy camper.

At home I tested and optimised my 40m balun. Ready for the final assembly and testing.

This weekend I had some time to go out to bring it all together and enjoy the fruits of my labour. However when I set up the antenna with balun I found.... it was way off. Go figure.

Tuning the C-pole yet another time

Discussing this with my YNOMY team members we came to the conclusion that the common mode current - considerable in this design - influences the measurements of the VNA. So tuning the "naked" antenna was a stupid idea and the time spent tuning was actually time spent detuning it.

A frustrated ham is never going to be the best person to work on any project but I did decide there and then to re-tune the antenna one more time. I was almost finished when I too hastily bumped the VNA against the pole. The center conductor of the SMA plug attached to the VNA broke off and is now securely in place in my VNA port - making it neither a female nor male connector (gender neutral - a modern concept). That concluded the tuning process for the time being.  

Gender neutral SMA connector

So now I have to replace the SMA connector on the VNA. That connector - I found - is soldered into place with lead free solder that I cannot remove with my soldering iron as it just does not heat up enough.
Luckily Marcel PG8M told me he both has the connectors and a proper soldering iron. So now I have to find the time to go over to his place and have him repair the damages (keeping a safe distance all the time of course).

Cooking on a choke

Out portable with my (relatively) new C-pole antenna last week I ran into trouble. After about 30 minutes signal levels dropped dramatically indicating something was wrong. I saw the SWR skyrocket.
Walking over to my c-pole I felt the common mode choke was boiling hot. So I changed antennas and continued my operation.

Back home I began my investigation in what went wrong. Immediately there were two suspects: the antenna (high swr for some reason) and the choke (too low choking impedance).

The antenna was fine when I used it the first time and I did not change anything in the mean time. So the choke seemed a more logical candidate. The purpose of the choke is to minimise the amount of common mode current that you will generate with an unbalanced antenna like the C-pole. If the choking impedance is too low however there will still be a considerable common mode current left that will generate heat in the choke.

W2DU style choke with ferroxcubes beads

On this latest homemade version of the c-pole I use a W2DU style choke that I constructed myself using a number of ferrite beads. The type and amount I used I based on the factsheet. On paper the choking impedance was OK but I can't remember if I really tested it. Checking it now I measured a whooping 400 Ohm of impedance - far too low to stop the CM current flow (I wonder if the heat impacted the ferrite?).

Removing turns of RG58 from two stacked FT240 toroids

On my first C-pole I used a different current choke, designed on the basis of the excellent information by the late G3TXQ. I used two FT240-43 ferrite toroids stacked with a couple of turns of RG58. It is bulkier and heaver though than a W2DU choke, that is why I changed it.
That toroid based choke also became very hot on my first c-pole but I later discovered the antenna itself had an issue - causing high swr - causing high voltages over the choke.

So I returned to this old choke - abandoned but not scrapped. I measured it and it had an interesting profile. It was particularly useful in the 80m band range (5k Ohm) but certainly not bad in the 40m range (2.5k Ohm).

Measurement #1 of the choking impedance and transmission loss on my balun

I decided to take it apart and remove one of the RG58 turns. As expected the choking maximum moved up. The profile now suited a 60m antenna perfectly while impedance was higher in the 40m band. After removing two more turns I got a maximum in the 40m band. The choking impedance is now more than 5k Ohm there. That should do the trick.

Measurement #2 after removing 3 turns of the coax

When I have the time I will go out /P with my C-pole to check it without a choke - just to be sure it still is resonant in the 40m band - and then add my old toroid based choke. 

A new addition to my portable antennas: 40m delta loop

Always looking for better antennas to use when portable, I stumbled upon the 40m delta loop. I had read about it a couple of times but mainly from expeditions using the delta loop on higher bands near to salt water. The results were very good but could not be reproduced on "regular ground". So I made a mental note to look into the antenna more later but without high expectations.

Then I came across an article by DJ0IP, who describes the delta loop on 40m as his favourite 40m DX antenna. Now that got my attention. Richard describes the loop and how to build it in detail. He opted for an antenna feed that makes this antenna vertically polarised with a low take off angle. 

The antenna does not need a lot of height to put up and looking at it from a /P operator, it is about as complex (or easy) to set up as an inverted V. So I decided I'd like to have one. I cut some DX wire to the right size for 1 wavelength on 7.1 Mhz. Then I created a 1/4 wavelength stub using 75 ohm RG-59. I never created a stub before but with the help of my miniVNA it was really easy to achieve the correct length.

Ready for transport: 40m delta loop with 1/4wl stub

This morning I had some time to try the antenna out in the field and tweak it to the correct length. So I went out to my favourite antenna testing field.

Delta loop in the field (lines indicate approx wire positions)

I used my 18m SpiderBeam pole and pushed the apex to about 14m. The center was about 4m high. By pulling both corners to the side I was able to create a triangular shape. Note that I attached the top of the loop to the third element of the pole. Attaching it higher I would pull the top too far down.

Checking the SWR I found I cut the loop too short - even though I had made it longer than the minimum length I calculated. So I added a bit of wire in the horizontal leg (as described by DJ0IP). I got the SWR dipping in the right part of the 40m band and still had a bit of time left to test it on the air.

WPX contest meant the band was filled with signals. Most of them were from the EU as I was nearing local lunch time. I did hear ZL4 peaking s7 outside the EU bandplan and I copied TI5 buried in QRM. So it did seem the loop was receiving signals. I tried TI5 (new one on this band) but I just could not copy him well enough through all the EU splatter. I also heard AJ4A in KY and he was able to copy me. Not a new DXCC but KY is the furthest I have worked into the US on 40m. So it seems the loop works both on RX and TX.
It needs a lot more testing to claim that it beats any of my other 40m antennas (inverted V, efhw, c-pole) but it starts out promising.

I did notice the antenna is very sensitive to height above ground (DJ0IP already warned me about that). So I added this feature to be able to tune the antenna quickly when using it again.

Copper wires added to the horizontal leg for easy tuning