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 remember in the 1970’s the heyday of CB radio, when I’d walk into the local Dick Smith Store and find just about any component I wanted. I also could see, on the shelf, equipment I aspired to own but could not afford. Online shopping and cheap electronics from China, have pretty much destroyed that particular market. Yes I can get some components from Jaycar thankfully but like the other stores, the range is getting more and more limited as the shelves fill with cheap (throw-away) electronic gadgets.
As I work through the repair of my vintage radios and amplifier, I am slowly building a tally of items that are almost impossible to purchase, have to be purchased in quantity from afar, or outright unobtanium. High voltage capacitors of specific values being one such example e.g. 360pF 1000V. Even if I search for options like two capacitors in series or two capacitors in parallel, it is impossible to reach the right values with what is available for less than the cost of a small motor vehicle.
Plate choke for an Ameritron AL811- temporarily unobtanium (RF parts and DXEngineering and others)
the aforementioned 360pF 1000V capacitor ( i can get some if I spend $35 plus shipping)
I even had trouble getting PCB mount 20mm x 5mm fuse holders, 8amp slow blow fuses and 15 Ohm 10W resistors locally.
I’m not convinced that online shopping is always convenient.
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 🙂
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 ÂĽ 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.
Those of you who follow my blog will be aware that I recently relocated and now have the opportunity to set up a more suitable shack, and have space for antennas. Some of my old and newer valve gear is coming out of boxes and finding a home in the shack. Despite more room than I had in Sydney, there is a distinct lack of suitable trees at my new QTH and I have yet to construct some kind of support for a dipole. I’m using a hastely constructed ZS6BKW that has a slightly shorter than optimal ladder line feed and as such can be a bit difficult to load on 80m. When tuning a valve amplifier this complicates things a bit more than is optimal, so I need a 50 ohm load to tune the amps and then I can switch over at low power to let the auto tuner do what ever is necessary on 80m to match the impedance. This is far better than trying to tune an amp into a badly matched load. Yes there are other ways around it but etiquette also says I really should not be tuning up on air if I don’t need to. A good high power dummy load, capable of handling max 400 watts is really what is required.
Given my budget, I really can’t afford to buy one, so I have plans here to build one. It will be an oil cooled 1kW unit in a paint can. I have to source the resistors. I previously showed a kit available for a lower power dummy load. The resistors I will need are these.
New paint cans are available from eBay. Using these, along with some copper plate and a few fittings, this should be relatively easy to construct. I’ll post the construction here.
Today after work, I decided to get the Drake TR-4C operating and connected to all the mod cons. I had to repair the interface for PTT, and mic audio which allows me to connect the Heil PR781 via the Behringer mixer, to the Drake. I then took the audio output from the Drake to the KRK studio monitor speaker.
I love listening to technical discussions on air and enjoyed listening in to a QSO on 80m, about the G5RV and ZS6BKW, which those of you who have followed me will know is a favourite subject of mine. The conversation was of great merit and interesting to listen to but as often is the case with these antennas, it perpetuated a couple of myths.
The ladder line of the G5RV and variants radiates- No it cannot. Think about it, the ladder line conductors are very closely coupled and out of phase, thus radiation is cancelled, unlike on the legs of the dipole which are also out of phase, but not closely coupled, they are heading off at 180 degrees to each other, not side by side at a tiny fraction of a wavelength.
The antennas are noisy due to the vertical ladder line. Only true if no 1:1 current balun is fitted. Again, think about it, the only noise that can make it back along the ladder line to the radio, is noise that is unaffected by the close coupling, ie the only signal that can make it back is common mode current.
“Common mode current is portion of conductor currents that are unmatched with the exactly opposite and equal magnitude currents. Common mode current cause multiconductors to act or behave like a single conductor.”
Unlike the radio signals which are Differential Mode (out of phase), CM current is directed in the same direction and results in a much higher electric field because fields from both conductors will be added
This is why Walter Maxwell and others, including myself, continue to say that a G5RV and a ZS6BKW, both of which are variants of the “random length dipole” absolutely require a 1:1 current balun between the coax and the ladder line.
You can build a random length dipole like a G5RV that does not suffer from noise, completely by accident, all you need is enough loss in the coax. A better solution is a 1:1 current balun. Additionally the 1:1 current balun reduces RF in the shack because the two legs of the dipole which are out of phase can result in a situation where, if the feedpoint impedance is high enough (and it will be on some bands), the returning current from one of the legs can return via the outside of braid on the coax. You see there are three potential current paths in coax due to skin effect.
The centre conductor (normal) connected to one leg of the dipole
The inside of the braid (normal) connected to the other leg of the dipole
The outside of the braid (not normal), making up a third leg to the dipole, potentially resulting in detuning (a tripole is not a very good antenna) and RF in the shack. When feedpoint impedance is high, the current takes this third route back to the shack. This is also the route common mode current (noise) takes.
So rule number one of the random length dipole is to use a 1:1 current balun at the junction of coax and ladder line.
The problem with living in Sydney is not just the issue of trying to build or purchase a HF antenna that can be fitted onto the balcony of a one bedroom apartment, but also the problem of an S9 noise floor. So it was that when I was asked to do some dog sitting at my RF quiet North Queensland residence, I packed a HF radio.
Of course, a radio is not much good without an antenna, but in my usual style I decided the best approach was to build one from scratch. I wanted to work only 80m CW and 40m phone and I could have chosen a dipole but the yard is a bit small. I had in the past constructed a 40m/80m trapped sloper referred to as a Lake Eyre Special and I figured this would be the ideal antenna. There are several tall Royal palm trees that would provide the necessary elevation.
I started by purchasing some speaker wire and splitting it into single wires then would 120 turns on to a 40mm pvc form. 10.04 m of wire formed the 40m section which I then tuned using my antenna tuner which I always pack in my holiday baggage :-). I then fitted the coil and double checked the tuning. A slight change to the coil of a few more turns had me back in tune. I then added the top section 1.6m and checked the tuning on 80m. Despite my error of tuning to 3.1MHz instead of 3.6MHz, I finally got things correct. The antenna was hoisted into the tree via the usual sling shot and fishing line method. I always carry my slingshot on holiday for this very purpose.
Shack of VK2KMI 