Monthly Archives: March 2009

Quadrifilar wound toroids (and Coax Cable stub filters)

quadrifilar wound toroidJames (M3YOM) has just emailed me after discovering something worse than fitting 90 or so PL259s last week; it’s winding quadrifilar wound toroids. Pictured is a completed L1 winding for the 40m filter. He says that it’s a T130-0 core with 7 quadrifilar windings which should give around 3.96uH and there are two in the filter box he’s making from a kit supplied by Bob Henderson.

Since James took on the job of producing a fully filtered Multi-Two station, he’s soldered nearly 100 PL259s and not one of them was faulty on the day. Good job OM!

James, the wire looks like that stuff we made the MegLoop with, is it?

James says:

“It’s not the megaloop wire, the Lo-Z winding (yellow wire) is 1.25mm solderable enamelled wire and the Hi-Z winding (Red wire) is 1mm solderable enamelled wire. The enamel is designed to burn away at low temperature so you can solder it directly without the need for striping it back first.

Incidentally, Bob’s callsign is 5B4AGN (he used to be a G3 before he moved) “

Bob, I’m sorry you used to be a G3. You should know that I’m forever poking serious fun at G3s, as they poke fun at M3s. Maybe you and I can form a truce and make the world a happier place?

73 all, particularly my new G3 friends.


M0XXT CQ WPX 2009 (Amateur Radio Contest)

M0XXT UK Scout Contest TeamMy thanks to Lee (G0MTN) James (M3YOM) Terry (G4MKP) and Aidan (M6TTT, Scout) for getting our new station on the air for a first-time-out on WPX.

We also roped in Chris and Dan, two new foundation student contest Scouts for the spotting on the Mult Station. Barry M0DGQ also gets a big thank you for supplying hundreds of meters of 75 ohm coax for James’s stub-filter project and Charles at Moonraker for sponsoring the large number of required PL259 and T pieces.

We ran a pair of very old Yaesu FT1000MPs (non-Inrad versions) which were completely swamped by QRM to a number of interesting antennas including our new super all-band Mega-Loop (horizontal delta loop at 85 feet) delivering about 12dbi on 20m to US at 10 degrees elevation (more on higher bands). However, it also delivers 10dbi to EU and most of Russia at a very low angle, hence QRM flooding. James’s stub filter project worked but still needs some tweaking. Thanks to K1TTT for helping James get his head around this. Next time, James will have finished Bob Henderson’s filters as well which should sort out any final interference between bands. Poor James soldered nearly 100 PL259s last week. Don’t worry James, fingers should re-grow by CQWW!

The mult station ran Terry’s A3S at 30 feet on my hydraulic tower and also switched in a 200 foot doublet at 80 feet across the tree canopy behind the Scout Hut for the low bands (which worked great on 40m DX but rubbish for 40m NVIS). Need some switchable low dipoles for EU.

We had an initial target of 2,500 QSOs and 1,000 mults and just missed both by a whisker.

  Band    QSOs     Pts  WPX 
   1.8     152     301   50 
   3.5     574    1513  313 
     7     866    2203  264 
    14     722    1378  269 
    21     102     254   63 
 Total    2416    5649  959 
Score: 5,417,391 

We had two breaks of 30 minutes each to sit and have dinner / lunch as a team, an important lesson to keep morale and spirits high. Some day we might not be able to afford time off, however whilst we’re still novices and part-time, we can afford to have a lazy lounge around. We sent our score to GETSCORES every 5 minutes automatically fed by N1MM on a local wireless LAN picked up from my house about a 900 meters away. S51A and ourselves played ping-pong on the scoreboard for the whole 48 hours and we’ve since become competitive friends.

We particularly liked GETSCORES since it’s a way of interfacing with the rest of the world and having even more fun. I don’t understand why more stations don’t use this system. The USA seem to embrace this stuff quicker than others.

There were many highlights. Working VK on short path AND long path on both days on 40m was a privilage. Many more experienced contesters might have done this before but at last a first for me. Having the young non-licensed Scouts successfully finding some RED mults on N1MM was cool. Being called in just to work them was even cooler! Handing the headset over and telling them, “Great. Find some more!” was even better than cool! James’s run into South America was nice on 15m on Sunday, as was the long run on 20m band. Being spotted three times in 5 minutes from the West then the East then the West again was like using a very high gain vertical (which was the bloody trouble actually since we couldn’t dial out the QR Mary).

Like all (good?) contest teams, we’re now working on massive improvements since the station only came together on the Thursday and we started transmitting essentially 90 minutes before the off. Everything worked, nothing broke. Amazing.



Transmission Lines

I have an article brewing in me which is all about matching a low dipole for 160m to regular 50 ohm transmitter. The dipole I had in mind will be about 15 meters off the deck. My impedance will be about 20 ohms, maybe 25 if I can get it a bit higher. Basically, this is unnaceptable to me since I like to match things nicely.

Chris (G0EYO) sent me this link which tells me that I need to insert a quarter-wave piece of coax that has an impedance of 35 ohms. Well, I don’t know about you, I don’t often have 35 ohm coax lying around, however a pair of 75 ohm quarter wave stubs will reduce the impedance of 75 ohms to 37.5 ohms. Near enough? I think so.

Thanks Chris, I think I worked it out now.


Kef 104/2

Kef 104/2 104-2 104.2If you have never heard about the Kef 104/2 (104-2) loud speakers, you have missed a life-time treat. I am lucky enough to have the gold plated, bi-wired version. In fact, I had the factory make these for me in the mid-90s, nearly three years after production ended since I missed mine so much. They really are the last pair of 104/2s ever made. Kef upgraded all the components for me and made me brand new Rosewood shells from new veneer. Fabulous looking.

Now. There was originally a weak point with these speakers which was the tweeter. It wasn’t the speakers fault, but the user. I used to blow the tweeters on my first pair of 104/2s too regularly, however replacements were about £15 each so I just used to order a new set and start again. Blown tweeters normally happened when the amp would clip over a long period of time and gradually heat up the tweeter coil. Two things might happen. The cylinder would either warp or the coil would melt. This was not a speaker issue but the user. Basically, turn the volume down. Easier to say than do since mid-party in the heat of the moment, it’s easy to over drive stuff (hence why I now run a 3kW rig, just for party nights – it’s true).

Anyway, let’s correct a couple of worries out there on the internet to do with the replacement tweeter issue. Last year, I thought I had lost a tweeter and called the factory. Contrary to popular opinion, Kef have sourced a fabulous replacement tweeter and they sell them in pairs very ecconomically for 104/2 speaker systems for about £30 a pair. I ordered them up and fitted them. It transpired that I hadn’t lost a tweeter, it was my graphic that had gone AWOL, however the new tweeter is better for a number of reason – not only do they sound great but apparently have better resistance to warping and coil metdown. These days, I just tickle mine with a Crown K1 and they sound marvelous. I ran them as a spare pair at a party last month in the conservatory at half-volume and a couple of people remarked how enormous they sounded.

If you want a speaker to sound four times the size that it really is, buy a pair of these, massive sound stage, big presence and even vertical sound balance. I have no idea how this happens but it does. Stunning. Did I mention massive low frequency response? Amazing for films and rock. Amazing for anything actually! You can pick up non-bi-wired versions from as little as £300 on eBay. The bi-wired versions were essentially upgraded versions with a bi-wire system which I call the Mk2s. These retailed for £2,000. Massive price back then! Starting price for used versions of the Mk2s will be around the £450 mark, perhaps up to £800 for top condition Rosewoods. Always check for matching serial numbers.


I sold these speakers a few months ago and it appears that the foam that held the bass cones to the basket had deteriorated completely and essentially fell off. He sent me the pictures. He discovered a man who rebuilt these at a cost of £85 each (x 4). I paid for them to be refurbished, otherwise the speakers would have been scrapped. They are now (apparently) as good as new. Now then, it appears that almost every 104/2 in the world will have the problem. What a disaster! Same with 105/3 and 107s too. A complete mess.