It is fair to say that I’ve been very happy with the performance of the Nested Marconi antenna to date, but I’ve not spent a great deal of time on bands other than 80m and 40m, where the nested Marconi performs really well. While the SWR is fine on all bands (except 160m), the performance has not really been explored by me.
Recently I tried it out on 20m, using the additional 10m/20m attachment and I found the performance much reduced compared to my ZS6BKW. 15m and 10m seem similarly reduced but I really need to do more on these bands to make a real assessment. .If you have a small yard, this is a great antenna but I’ve found it lacking on 20m. Today I made a coil for 160m (44mH) and it does allow me to tune it to 160m. I’m yet to test it out.
Today I spent some time modifying the audio interface that I used to use with my Icom IC706MKIIG, so that it would work with the IC756ProII. I removed the 13pin DIN and replaced with with an 8 pin microphone plug. Unfortunately I didn’t’ have much cable so I had to make do with what I had. The box contains two 6.5mm phono sockets wired up to the microphone plug. One socket is for PTT, the other for audio. PTT is operated via foot switch. It is important when setting up something like this, that you adjust the microphone gain, and mixer so that your ALC is as per the manufacturer’s recommended settings.
I’ve been off air for some time because I’m restarting my business, Learn to Tow at http://www.learntotow.com.au. I deliver towing training on weekends. I’ve also got a new full time job at Cessnock, so once the COVID situation changes I’ll be moving house. Meanwhile the Barrett 950 had to go into the 4WD, so I needed a new radio. I managed to get my hands on an ICOM IC756ProII which I’m loving. It is now time for me to finish off the repairs and modifications to the Ameritron AL811.
“Always walk through life as though you have something new to learn, and you will.”
Due to delays waiting for parts for my Ameritron AL811 project, I decided to start on another repair job that has been awaiting my attention. Last time I used my Icom IC-703 there was no power out on transmit. I suspected blown finals- that will be a painful repair job with SMDs! But today when I attempted to power it up, it was completely dead. I opened it up and checked the internal fuse; it was intact. I checked the rectifier diode a DSA3A1. It was dead. Hmm Why a rectifier diode on a dc supply? I guess it is normal to use these where there may be a fairly decent current (direct connection to a car battery for example) and to provide reverse polarity protection from that current.
So OK but why no transmit when it was powering up? A closer look revealed a little teeny resistor that runs from a track on the PCB, to the centre of the antenna socket has come away from the PCB. It had broken off and left the tail embedded in the board. This thing was tiny! I had to desolder the antenna socket anyway, to get access to the bottom of the board, to remove the diode, so I set about doing that and flipping the board. That is when I saw this tiny resistor. It had no coloured bands at all but it looked to me to be a resistor. I searched the Icom Service Manual and found it on the PCB drawing, labelled W999. The table of components told me it was generally referred to as a Jumper ERD-S2T0. Searching this info online I found that it referred to a zero ohm resistor. Now that got me really thinking. What possible purpose could be served by a zero ohm resistor? Why not just a short wire?
The answer may well not surprise you, it didn’t me. The answer is speed, time and cost in manufacturing, especially if that manufacturing is done in part or completely by robotics. The part has no electronic purpose.
If you have been following my posts, you will know that I’ve been working on an Ameritron AL811 which recently experienced a flash-over and took out the Plate Choke among other things. Now, with this amplifier and 811 valves, many would be quick to point the finger at the poor old 811 weaklings, but that may not be fair. So here is a little more context-
I recently moved out of the city to a regional town and finally was able to play amateur radio again. I had room for antennas and I was away from the horrible QRM of the city. Unfortunately my block has few trees to hang antennas from and I had zero funds, so I cobbled together the only antenna I could from the bits and pieces I had, a nested Marconi- See previous post. The antenna performs beautifully once I spent a bit of time getting the element lengths and counterpoise length, height and insulation correct. It was resonant on most of the 80m band, some of the 160m band and all other bands bar 30 m. But you wouldn’t know that from my LDG AT-1000ProII.
Don’t get me wrong. I am not knocking the LDG tuner, it’s a great bit of kit, but it is known to suffer in extreme circumstances from RF interfering with the microprocessor, thus display incorrect SWR indications on the LED bar. This was the case for me, as I always checked SWR with my Diamond SX100, which despite the name is good for 3 kW. The tuner was not really needed, as I could mostly keep it bypassed, because I had good SWR on all frequencies I used. But as I would find out, there is a catch.
When I’d installed the antenna, the existing high point to which I could attach the 80m leg was directly overhead the shack immediately above the point where the coax entered the room. Literally within meters of the radio. This of course meant that with the amp running, on 80, even without it running, I was getting RF into the shack, not via high SWR or high feedpoint impedance, just pure good old radio waves. Now of course the valves in the amp don’t really care how the RF get’s into them, they just know how to complain when it does.
So today I moved the antenna. It took a bit of work as I had to relocate the weather station and fit a mast to the bargeboard, but I got it done. Now I await the parts for the amplifier. The RF choke could be easily rewound but the replacement part is $30. I’ll have a go at rewinding it, at some later date. Beware overlaps in this choke, that may have contributed to the early failure. Ameritron skimped on the varnish, so many of these have loose windings that droop down and overlap the windings at the top. This WILL lead to a failure eventually.
So whatever you do, keep the RF out of the shack, even if it isn’t coming back along the feedline 🙂
WARNING: LINEAR AMPLIFIERS CAN RETAIN LETHAL VOLTAGES EVEN WHEN TURN OFF AND UNPLUGGED- YOU HAVE BEEN WARNED
Recently I purchased an Ameritron AL-811 amplifier. This is the 3x811A setup which Ameritron claims is good for 600 watts PEP. I think that is stretching things but that’s a whole different write up for another day. This one was in beautiful condition but appeared to have some issues that led to the destruction of an 811A valve (we don’t call them tubes here in VK) when I was using it on 80m. It also blew the D16 diode which I replaced.
I was pretty new to this amp but know the specifications of the components pretty well. I suspected a padding cap had failed. I find now that on 80m the load is at zero for best power and that amounts to less than 300 watts. Load at zero is never a good thing. So I now have ordered several replacements, there are two used on the 160 and 80m bands. For now I am staying off 80m. The amp is working beautifully and has allowed me to make some great DX contacts from my new QTH including a new DXCC Montserrat. The nested Marconi seems to be a great antenna too. I may just keep it. 🙂
I’m really looking forward to using this amp on AM with the Johnson Viking Ranger II. Many people claim that the amp isn’t good for AM, but that simply isn’t true in my view. At 100 w carrier, my legal limit for AM, I can run it on 40 metres with 180 mA of plate current. The max specification for high duty cycle modes is 400 mA. Of course, to use it with the Viking Ranger II, I’ll need a 6dB attenuator which is on order. I may even need the attenuator to be 10dB, we’ll see.
When the caps arrive, I’ll replace the 80m cap and see how we go on 80m. I also have three 572B’s on order, as this amp will ultimately be changed over to those, once I have the bugs ironed out. They are much more robust, but if there are faults, I’ll kill them too, and I really don’t want to fry $350 worth of Chinese glass.
Back in 2013 I suffered a massive financial loss. I ended up homeless after running a successful business and owning two properties, one on acreage, where I could have pretty much any antennas I wanted. Despite all that room, my partner was keen that i didn’t cover the whole yard in antennas, so apart from my 160m loop, and an 80m delta loop, my favourite antenna was a ZS6BKW, I had two of them.
Later when I moved to Sydney, I kept a ZS6BKW in storage but never had the room to install it. Somewhere along the way it got lost. At some point I hastily constructed a random length dipole for SOTA and VKFF (WFF) and by the time I got around to looking at it for use at my new QTH, I had cut some of the 450 ohm ladder line off it, leaving it with just 9 or so meters of ladder line, which unfortunately does not correspond to any dipole design. I also had no poles to hand a dipole from. I could attach a pool cleaning wand to an existing TV antenna mount on the house and I had a 12m squid pole. So it was that I started searching for an antenna design that I could build with my scrap pieces of wire and the squid pole- enter the Nested Marconi.
The Nested Marconi design could be build with just 7.5 m of 450 ohm ladder line and would require no more wire than I had in my possession. I could attach the centre to the 12m squid pole, the short horizontal to a tree and the long horizontal to the pool cleaning pole mounted on the house. All attachments would be done with 150 pound fishing line. I added a 20m and 10 m element too. Next write up, I’ll tell you all about what you may encounter if you decide to build one and I”l show the photos of just how ugly it is :-). It would not be difficult with a better set up to make it more attractive 🙂
For some time now, I and many like me have been restoring older radios using NOS (New Old Stock) valves, what you Americans call tubes. Tubes here contain beer, not cathodes and anodes and screens and grids in an absence of air. Recently I had an 811a valve/tube fail in my Ameritron Al-811. The failed item was a Matchlett which frankly I think are no different to any of the other Chinese made valves available today, so I ordered some Shuaguan 811’a from China.
As is often the case with my little online automotive and small engine parts business, the Chinese supplier turned out to be a reseller more hopeful about sales than actually equipped to proved sales. I cancelled the order and sought a new supplier. Sure I can buy Matchletts online from the USA if I want to pay $150 postage on top of the purchase price,. I’m at least likely to get them, but I’d get 572bs from China for the same price. Which is my next order. The Ameritron AL-811 works better with 572bs anyway, so my next order will be those. I just need to be sure the recently bought amp has no further issues before I stick $350 worth of Chinese glass in it. Oh for the old days.
Moving to a regional city has has been great for my ham radio but I’ve not been able to capitalise on it all that well, because of the antenna I have. I can’t seem to locate what I did with my stainless steel ZS6BKW which is a great shame. I doubt I threw it out in my moves over the last 6 years but maybe I did.
Hastily made for WWF and SOTA, the antenna I have has a too short ladder line for the design and in my new location, I just can’t tune a match, not even on 40m. Having spent $5000 moving, I can’t even afford a bit of wire, so I have to work with what I have- I’m guessing, about 9m of 450 ohm ladder line and about 30m of heavy duty speaker wire (what the current antenna is made from) and possibly a bit more wire somewhere.
So while I wait to get a few more dollars in the bank, there are a couple of options I can explore, a nested Marconi or an inverted L. For now I’m going to try the nested Marconi because I have what I need for that.
This antenna was first conceived as a simple dual band antenna in 1988, in an attempt to achieve sensible dual band operation in a small garden of 14m (46 ft) length.
Evolving from a simple end-fed Marconi for 80m, the idea was to use the opposing harmonicrelationships of two ¼ wave elements so that they could be fed on odd harmonics without mutual coupling.
This principle has been used with nested dipole antennas and other specialist multi-band antennas such as the Cobweb, but because of interaction and coupling issues, it’s rarely implemented on end fed wire antennas. With this version of the antenna, interaction between elementsand top resonance have been turned to our advantage allowing for excellent multi-band working, while maintaining a useful radiation pattern and efficiency on almost all Amateur Radio bands.
The suggested dimensions provide for low radiation angle (30 to 40 degrees) on all bands except 30m where the antenna acts as a full size horizontal dipole.
Low VSWR is achievable on most bands and, with careful construction, up to four bands can be used without ATU.
Radiation resistance is in the range 25 to 200 ohms on all bands, ensuring high radiation efficiency, even with moderate earthing arrangements. Unlike the Windom or G5RV, the fundamental bands can be resonated independently.
Shack of VK2KMI 