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What Purpose the Zero Ohm Resistor?¬† ICOM IC-703 Antenna Socket

March 24, 2021 Leave a comment

“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.

Categories: Daily News, HF

SWR, Valve Flash-overs, Antenna Position and RF in the Shack: It’s not always what it seems

March 21, 2021 Leave a comment

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 ūüôā

73 de VK2KMI

Categories: Daily News, HF

My Nested Marconi- Ugly but Effective Multi-band Antenna

March 6, 2021 1 comment

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 ūüôā

Categories: Daily News, HF, Home Brew

Time for an Antenna Change

February 4, 2021 Leave a comment

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 ¬ľ 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.

Details can be found here

Categories: Daily News, HF, Home Brew

Antennas, Apartments and AM Reception

February 5, 2017 1 comment

For some time now I have struggled with the fact that no matter what antenna I use, they couple to the high rise building in which I live and are quite deaf. Today I draped the Lake Eyre special antenna off my balcony as far as I was game. The building management would come down on me if they saw it. It’s better but still quite deaf. This is my Hammarlund HQ170 listening to the ARNSW broadcast on 40 meters AM.¬†Screen Shot 2017-02-05 at 11.09.54 AM.png

Categories: HF

Antenna Plans @ VK2KMI

October 2, 2016 Leave a comment

Following on from

80m Apartment HF- The Monster in My Bedroom

Taming the Monster

The Monster is Deaf

The Monster will Live Again

The 10kV capacitor arrived a few days ago so soon I will have to begin modifying the 80m and above mag loop. Probably over the next few weeks, as I quite a bit of travel to do. I am still concerned though that I will have to get it away from the building and over the balcony rail for it to work well. This high rise building is so full of steel and the balcony is framed by aluminium windows. The concern is, if I do that I am likely to attract the attention of the building management with such a large copper loop. The capacitor is huge and heavy so mounting it at mid point of the loop will present some issues, not the least of which will be drawing further attention to the thing hanging over my balcony rail.


My intention at this point is to mount the capacitor lower down on the support frame and inside the line of the balcony rail and use RG213 coaxial cable to connect it to the loop. This will add a small amount of capacitance which may affect use on higher bands. I guess I will just have to try it and see what happens.





Categories: Uncategorized

Fine Tuning the 32.3 ft Vertical- A better Antenna for 15m

December 11, 2013 Leave a comment

So my last post, described a better 15m vertical, which I had some success with. It tuned up where I wanted it to, but I had not had the chance to check it with the MFJ259B analyser. Though intended for 15 meters, where it is a 3/4 wavelength,  it is designed to be resonant at the lower part of the 40m band, where it would be a 1/4 wave. I finally got the opportunity to test it and I found it resonant at 6.8 MHz rather than at 7.000 Р7.010 as planned. I checked it at 21 MHz and found it resonant at a lower frequency- not what I wanted. I fiddled with the antenna length but just got nowhere. I then thought maybe the long 2 metre aluminium section of the base of the vertical which was now part of the radial system was somehow changing the resonance. So I lowered the assembly so that the feed point and radials were closer together and restablished the calculated vertical element length (after trimming it previously).


Scan 1

With this lowered feed point, I was finally able to get the antenna resonant approximately where I wanted it to be. The results are below.

Freq MHz   R              X          SWR

7.000         31              0           1.2:1

7.300          32             0           1.2:1

21.000       125            0           1.5:1

21.340       133            0            1.6:1

And just for good measure

18.068      42             16           2.2:1

18.150       51              19           2.2:1

24.890      23              0            1.9:1

24.950      25               0            1.9:1

28.000      166            0            1.9:1

29.300      21               0            1.9:1

Categories: Daily News, HF, Home Brew

Time for a Better 15m Antenna

December 10, 2013 2 comments

Over the weekend I was disappointed that I could not contact any of the many SKCC WES stations on 15 meters because I could not tune any of my antennas for 15m. My 80m loop was on the ground, and nothing else would work on 15 meters band. Until I get the tower up and the StepIR fitted to it, I have to rely on my 80m vertical loop for 15m. While it has worked well for PSK on 15m to Asia and Europe, it has not been much use for other directions, and it has a tendency to spend more time on the ground than in the air due to its tree top mountings. ¬†So having a useable antenna for 15m is a high priority. When I made my 43ft vertical, I didn’t really consider whether it would be any good on 15m, and as it turns out, I can’t get a match on 15m. While investigating other antennas for 80m, I accidentally came across an article titled A High Performance 1-wire DX Antenna which describes a vertical antenna of 32.3 feet length. At this length the antenna works well on 40m, 20m, 15m, 17m and 10m.¬†Since I had reason to pull my vertical down for repairs, I decided to test this 32.3 foot length.

My vertical uses a Spiderbeam collapsable fibreglass pole. The vertical element of total 43 ft consists of aluminium sailboard mast and a wire running the length of the fibreglass pole. It is fed with a DX engineering UNUN designed for their verticals. I also had a trapped vertical for my work radio on this pole, and decided to remove it, as I no longer have use for it. The spiderbeam pole is I think 12 meters long and is mounted on about 2m of aluminium sailboard mast which is in turn attached to a 2 metre high timber pole concreted into the ground.  Thirty-two elevated radials about 1m above the ground radiate out from an aluminium ring on this timber pole. The aluminium  sailboard mast was part of the 43 ft vertical antenna element in this installation.

With the wire cut to 32.3 feet, I now had excess fibreglass pole, so I decided to use the aluminium sailboard mast as part of the radial system rather than as part of the vertical element. This would allow me greater overall height. So I attached the UNUN higher up the pole at the base of the fibreglass and connected the wire element directly to the appropriate terminal of the UNUN. I then used the woven copper metal strap (supplied with the UNUN) to connect the top of the aluminium sailboard mast to the UNUN . The other copper strap which had been used to connect to the vertical now connected the base of the sailboard mast to the ring for the radials.

What I now have is a vertical antenna that is resonant at the bottom of the 40m band and has theoretical SWR of 1.7:1 at 7300 kHz ( we will test later), it is a 5/8 wavelength vertical on 17m (which I can’t use yet) and 3/4 wavelength vertical on 15m. Now I know a 3/4 wave antenna is going to have a high take off angle but it beats my current setup.

I made a few hardware modifications to ensure minimal maintenance in the future and then raised the antenna ¬†for testing. I’ll use the antenna analyser later, but for now I just checked that I could tune on all the bands I wanted, especially 15m, which it turns out, I could do. ¬†Antenna analyser results will be added to this blog later.

Spiderbeam pole attached to sailboard mast - note traps as well as continuous wire- two antennas in one originally.

Spiderbeam pole attached to sailboard mast – note traps as well as continuous wire- two antennas in one originally.IMG_0573


Unun connected by copper strap to sailboard mast which is part of the radial system.

Unun connected by copper strap to sailboard mast which is part of the radial system.

DX Engineering Unun

DX Engineering Unun

Categories: HF, Home Brew

How to Build a ZS6BKW Antenna (optimised G5RV Antenna)

September 2, 2013 7 comments

The ZS6BKW antenna is a better match for our bands than the G5RV. The method of construction and tuning  is essentially the same for both antennas but the dimensions are different.


The ZS6BKW is a “random length dipole” with ladder line acting as a 1:1 transformer. I prefer to use 450 ohm 18 AWG solid ladder line which is readily available on ebay in appropriate lengths (20m). ¬†The antenna is designed ¬†with the dipole arms electrically 1.35 wavelengths. The cut lengths must correspond to this ¬†(allowing for velocity factor). The ladder line should be 1/2 wavelength ¬†(corrected for velocity factor).

You will need

  • 1 only 1:1 current balun
  • 1 only 20m roll of 450 ohm ladder line (window line).
  • 2 only lengths of wire for the dipole arms (about 14.8 m each to start). I use stainless steel woven wire for strength.
  • 1 only HF antenna analyser- if you are intending building your own antennas, this unit will be essential.
  • 1 patch lead with suitable connectors to go between your antenna analyser and your 1:1 current balun (usually PL259 connectors)
  • 1 only 50 ohm resistor (not wire wound)
  • soldering iron and solder
  • Means to cut the stainless dipole arms.
  • Side-cutters ¬†to cut the ladder line.

The first step is to get the ladder line to the correct length to act as a 1:1 transformer. This best done with an antenna analyser and a few bits and pieces. Cut your ladder line to 13.11 metres length (43ft). solder a 50 ohm resistive load across the two leads at one end of the ladder line. Do not use a wire wound resistor. Connect the other other end to your 1:1 current balun. I like to use solder-on ring terminals for this job.

In order to determine the velocity factor of the ladder line, so that you know what length to cut it. Calculate the half wavelength frequency and compare it to the measured frequency.

Measured frequency- Using your antenna analyser connected as described, determine the frequency at which the impedance is 50 ohms – usually around 12 MHz for typical ladder line of this type.

Calculated frequency- 1/2 wavelength in free space corresponds to 150/WL = 150/13.11 = 11.44 MHz.

Determine the Velocity Factor of the ladder line by comparing it to the free space calculated frequency. e.g. If you measured that 50 ohms occurs at 12.54 MHz with your analyser, then the VF = 11.44/12.54 = 0.91.

My measurements gave a VF of 0.891 for my ladder line. I then multiplied this velocity factor by 13.11 to arrive at the length to cut the ladder line. = 11.68m. Thus the ladder line is now the correct length to provide a  1:1 match with the antenna.

Now to the construction of the two dipole arms. You should at first cut these to a fraction longer than 14.46m (47 ft 5 inches). I add about 0.3 m to each end and cut mine to 14.76m. Making them a bit longer, to begin with, allows you to double back the ends to adjust length and to provide means to attach an egg insulator. I adjust the electrical length by doubling the uninsulated stainless steel wire back on itself and use stainless steel cable clamps to hold it.  Use a suitable insulator piece such as perspex between the two arms.  I connect the stainless steel wire to the  ladder line using ring terminals and stainless steel bolts through the perspex.

Hoist the antenna to its final height and test SWR at 14.200 MHz. If the measured SWR is lower at a lower frequency, the antenna is too long.

Once you have it to the correct length, connect a suitable length of coax to the 1:1 current balun and to your radio and get yourself on air. The antenna is good for all 80m, 40m, 20m, 10m, 6m but not for 15 m.


Categories: HF, Home Brew Tags: , , , , ,

The G5RV and ZS6BKW Antennas Part III- A summary

If you arrived at this post directly, please go back and read Parts 1 and 2 written quite some time ago. ¬†I suggest anyone reading this post, obtain a copy of Walter Maxwell’s excellent book Reflections III. In it he explains the misconceptions about a G5RV and provides excellent information. The book is available here¬†

Firstly let me summarise the key learnings about a G5RV antenna from my previous posts.

1. The G5RV is a random length dipole just like any other random length dipole.

2. Varney (G5RV) designed the antenna to have multiple lobe radiation pattern on 20m.

3. Varney intended the antenna to be fed with open line feeder from transmitter to antenna.

4. The G5RV if built correctly will have an SWR of 1.7:1 at the resonant frequency on the 20m band. SWR will be very high on other bands.

5. A transmatch (tuner) is required.

6. The open wire feeder is cut to a specific length so as to act as a 1:1 impedance match ie. the impedance of the antenna will be seen by the transceiver.

7. If feeding with coax to the open wire feeder, a 1:1 current balun is required at the junction of open wire and coax. ie. where you go from BALanced to UNbalanced. Without this the coax will act as a third component of the antenna and will radiate, reducing efficiency, increasing receive noise and causing interference.

8. The ZS6BKW variant provides a better match on most modern bands and is what I now use.


Categories: HF, Home Brew Tags: , , , ,