Wednesday, April 29, 2020

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).

Saturday, April 18, 2020

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. 

Tuesday, April 14, 2020

Recent PAFF activities

Covid-19 has motivated me to go out portable again after months of little to no activity. Apart from some field tests I went out for two PAFF activations. One to PAFF-0016 (Sallandse Heuvelrug) that I visited once before in 2012 and one to PAFF-0055 (Deelerwoud) that I had visited four times already as it is near to my QTH.

The PAFF-0016 activity was on the Saturday afternoon during WPX SSB. There was no choice in the matter as I was out with my family and there was only that particular afternoon available. I was doubtful about my chances to work a nice number of stations (my target is a minimum of 100), but it was even worse than I feared. In 2,5 hours I only logged 26 QSOs from 13 DXCC. That is by far the lowest number I have logged during a WWFF activity.

It started out alright with K2VV (Missouri) as the first chaser coming back on 20m but then the contest QRM became stronger and stronger. The higher bands were closed (4 QSOs on 17m in 30 minutes), so I tried some (fake) CW on 20m. That gave me a few new chasers but that was slow with 10 chasers in 30 minutes. In the end I went down to 60m to add another 7 QSOs before calling it a day.

QSOs from PAFF-0016 - nicely spread out but only a handful

My visit to PAFF-0055 was the next Saturday and was completely the opposite. There was a contest that weekend but it would only start at 16h UTC. That gave me some time to use 20m and 40m before QRM started. 
The 6th chaser to return my call was Norm N9MM from Texas. That was good propagation news as TX is not regularly in my /P logs. Two more DX surprises were a new JA chaser and Paul VK5PAS who I had not heard for 3 years. 
Europe was well awake and the skip zones of 20m and 40m overlapped nicely. It was busy throughout. I logged 156 QSOs from 32 DXCC in just under 2 hours even though I had to change antennas for 40m as I seemed to have burnt the common mode choke of my C-pole...

QSOs from PAFF-0055 - activity level that makes an activator smile
That last activity was motivating which is good because I have a busy month of May ahead. I will be active with a special call - PH75FREE - commemorating 75 years of freedom (after the end of WW2). I hope to do most of the activity /P.

Sunday, April 12, 2020

Up a tree - part 2

After dusting off my EZ-hang slinghsot I replaced the old and in the meantime shortened fishing line with new fluorescent line (0.25mm). I started out with 120m but that was a little too much for the reel so I cut it back to 100m.

I was wondering what the effect of the different sinker weights would be in practice - which one would travel higher and how would they behave once up in a tree.

Slingshot with 1.75oz sinker
Slingshot with original sinker

First I did some simple shots across a field. It turned out both weights ended up at around the same distance: 50m (170ft) with a maximum of 54m for the 1.75 oz weight and 58m for the original 1oz weight. I would say comparable.

Lots of nice branches
Then I selected a few nice trees to compare the two sinkers further. Again the difference in height reached was not really significant. Both ended up around 25m (80ft) with the 1oz weight reaching the highest branch at 28m (90ft). This corresponds to the highest mast I have (26m) so more than acceptable.

Two differences were significant to me:

1. the original (yellow) sinker was easier to find (higher visibility thanks to the paint)
2. the heavier sinker came down smoothly every time while the original one had to be helped down more than once (tugging the line)

So I think it is a matter of painting the heavier sinkers and I am good to go.

Another thing I will optimise are the rubber bands of the slingshot. They can be stretched quite lightly even up to my maximum reach. As it turns out (as with everything you dive into) there is a whole unknown territory to explore when it comes to designing good slingshots. Another thing to put on the to-do-list. On top of that list is figuring out why my 40m c-pole antenna kills current chokes. More on that later...

The "chosen one" in action

Friday, April 3, 2020

Up a tree

Being confined to home during the whole week - working remote through video links - I was looking for an outdoor activity. I decided to do some testing on my various options of launching wire antennas in a tree. Being able to deploy an antenna at a decent height without a mast increases the options for effective activities /P. 
In my little test this Sunday I used two different weights and my slingshot (by EZ Hang).

When I started looking for ways to get a wire up a tree I tried various objects and lines. I found out quickly that it is important that the wire is smooth - so fishing line or decoration line (used in shop windows for example) is the way to go.

Decoration and fishing line

As to the object to throw you need something that is easy to handle, smooth / round (so it glides through the branches) and a convenient throwing weight. Regarding the weight you are looking for a sweet spot: heavier and it will get difficult to throw high, lighter and the object will not return your line back to earth but just keep on dangling somewhere high up in the tree. In that situation you are left with tugging the line hoping the object continues its descent but in my experience you might just as well cause the object to swing around the nearest branch - leaving it behind for eternity. 

I started out with a ball as throwing object. Trying different types I settled on a field hockey ball. The reason being that it fits my hand perfectly and the weight is around the sweet spot (150gr) - not too heavy but generally enough to bring the line back down. Just drill a hole through the middle and away you go.

The ball is kind of large though so not so practical to carry around. Later on I started using fishing sinkers, settling on the heaviest one I could find (90gr). 

Throwing weights: hockey ball and fishing sinker

I tried both and measured the height I could get a line up. Of course it is slightly unscientific as I used only one tree in this experiment - so not all heights were achievable only specific branch heights. It turns out I can throw the fishing weight just a bit higher than the ball. A few tries brought it up to 12 meters / 40 feet. A decent height for a wire antenna on the higher bands.

EZ Hang slingshot

Then I turned to my slingshot. I have been struggling with it in the past, getting the line stuck in the reel or not having the ball come down from the tree (losing a lot of line).

After a few tries I did manage to shoot the provided metal ball a nice way up and through the tree reaching a height of 25m. I think I can shoot it higher than that if I give it a bit more practice. That is a good height even for wire antennas on the lower bands.

The EZ hang standard weight - a 25gr canon ball

The main weakness to me is that the weight of the standard ball is too low to be sure it comes down from a tree with a lot of branches (let alone leaves). So the next thing I will do is test this out with a few different weights. I also have to replace the line as it has become too short after losing parts of it in the past (stuck in trees).

Will share more experiences in the future.