Saturday, 28 June 2014

Transportable WiFi Repeater

Testing the Chinese import WiFi repeater yesterday revealed a few good features, but also one very major and disappointing one. On the plus side, it is actually rather easy to set up, although it is easier to do so using the RJ45 LAN port than by wireless, if only due to the fact that as soon as its connected to a network, it becomes 'transparent' and cannot be 'seen' by the devices on the network! This means you cannot reconfigure it without a hard reset, as you can't access the devices IP address!

The big downside is its power supply. Physically, the two pin mains connector is very poor, and is so loose in an adapter for UK mains that just knocking the device causes it to reboot. I feel that this is a dangerous situation and have complained to the supplier.

Today was Toms birthday party, and with less than an hour to go, we found that the professional audio system we had for the music would not work. It seems it runs on internal batteries, and these were dead. Both the mains and the DC power input are charge circuits, neither bypasses the batteries and allows the unit to run! It is also a 24V system. So I had to desperately strip the unit, disconnect the batteries, find a way to bring the cables out, and rig a temporary 24V DC supply

The spade terminals I had were too big, so I had to cut them down using tin shears. The power came from two 12V 38Ahr SLABs in series.

In order to build the proposed transportable WiFi repeater, I need some suitable coax connectors to allow connecting various 2.4GHz antennas. Notably, in order to use collinears or Yagis. The old Hirshmann TV transposer cards on my shelf yielded two short SMA coax leads, as seen in the photo below

 These are ideal. All I need now is a 12V to 5V regulator board, a 7Ahr SLAB, and a suitable metal box with a handle to build it into!

I have also finally installed the coax switch in the shack. This is mounted above the ATU and is between the ATU and the transceiver. It will allow me to connect other radios, in particular my MKARS80 and the PSK-20, to the main antenna. This switch has a center position which puts everything to ground as well, so when im not present I can ground the station and antennas for safety.

I still need to find an old PS/2 mouse (or keyboard) to yield a data cable for my FT-857D, plus a spare USB lead to finish the USB soundcard that will be boxed as part of this interface.

Thursday, 26 June 2014


One of the challenges of the CHOTA station is that in order to have live maps of WSPR spots, I need an internet connection. I could use my phone as a Wifi hotspot, but the GSM/GPRS signal is almost none existant at the church. The local pub is about 200m away and has free wifi, but its out of range.

What you see in the photo is a low cost, far east, WiFi repeater! I took a punt on this device, with the fear that it would use a single PCB mounted antenna, which would prevent its modification for greater range. When it arrived, my second worry was that its two pin mains connection, which I simply will not trust to be safe, would go to a SMPSU which would be on a single PCB with the repeater circuitry.

My fears seem unfounded. On opening the box up, I find that the SMPSU is on its own board, and simply feeds 5V DC to the main circuitry. Excellent - remove that and add a 7805 regulator, and it will run off a 12v 7Ahr SLAB. Also to my great relief and pleasure, it has two antennas - connected by coax!

The plan is to replace the antennas with coax sockets, and use external WiFi antennas (I already have a 2.4GHz Yagi), add the regulator and SLAB (Sealed Lead Acid Battery), put it all in a box, with a handle, and hey presto! A transportable WiFi repeater node!

I have yet to test the unit, and its always a bit hit and miss with these far east made cheap gadgets. I'll report back its performance later.

The extra 300 ohm window line has also arrived. This means I can finish the feeder section of the ZS6BKW antenna, and also make another 2m Slim Jim. I intent putting this one inside the radome of a defunct Jaybeam colinear that I have kicking about the workshop.

Tuesday, 24 June 2014

Well Thumbed Nuts

Actually, they're bolts, not nuts! The securing bolts holding the FT-857D to its mobile mount have always made it awkward to remove the rig for other use than mobile, so today they were replaced by nice shiny thumb screws. I'd been meaning to do this for a long time, but the necessity of using the rig for CHOTA pushed me to get the job finished.

On the subject of CHOTA, I have a confirmed joint meeting with the Trust and the landowner on monday. Also, I have the license! Ofcom, having required me to post the form to them, emailed the license back to me! So we are good to go as GB0SML in september.

I now need to get the FT-857D set-up for WSPR and/or PSK. This means yet another interface. The rig uses a 6-pin mini-DIN plug for its DATA port, which also provides fixed level audio out, and an AFSK audio input. There is also a PTT line, but the radio also has a 'Digi-VOX' option, so no PTT line is needed as the radio will sense the Tx audio and go to transmit. I just need the plug and cable, which means finding an old PC mouse to dismantle. Then, using the company Wifi, the rig, mobile installation and laptop, i'll test WSPR out whilst at work.

The hope is, that on the day, WSPR will be running on 10m using the fibreglass mast and 10m J-pole, and connected to the internet via the local pubs wifi!

Saturday, 21 June 2014

FT-857D for WSPR?

Its quite obvious that i'm not going to complete 10m Wispy in time for the CHOTA station, so have decided that the FT-857D should be used as one of the radios for the day, along with the Alinco DX-70TH. What I havent yet decided is which one for SSB and which for WSPR!

On the one hand, I already have a datamodes interface for the DX-70, but, I dont have a control cable for the FT-857 for the RT-11! I do have one for the Alinco! I am also much more at home with the Alinco for SES and contest work.

I believe however that the Yaesu has a dedicated datamodes port, which will make setting up for WSPR much easier. So with this in mind, today ive ordered a set of M4 x 6mm thumb screws. The reason being that these will make it a damn sight easier to get the set out of the car!

Ive also ordered the extra 300 ohm ladder line needed to complete the antenna. The extra (Im 1.1m short, so have had to order 5m!) will go to making a new 2m Slim Jim, to be encased in the tube and fittings of an old UHF colinear.

I just need the 4lb sledge hammer now and I can rig up the antenna to test.

I also now have an onsite meet arranged with the landowner next monday, all being well, and a plan to use a WiFi extender to 'borrow' the Crooked Billets internet for the day!

Ive spent some of today continuing on an audit of my coaxial feeders and patch leads. Each one is measured and gets a tag showing what cable it is and how long, this will save having to unroll them only to find its too short for the job! The audit sounds an easy job, but two of them were 65ft and 75ft lengths of LDF2-50 Heliax! That stuff doesnt like being coiled up!

I have also dismantled 'shorty', and taken the fittings and attached them to a much longer fibreglass mast section, ready for the next attempt.

Sunday, 15 June 2014

Shorty and Stealth

The 'Selby Shorty' mobile antenna has been analyzed, and found to, as expected, not be very good! Although it does have a resonance just below the 20m band, the radiation resistance is so low its really not much use, and the SWR on 20m is just beyond whats really usable with the FT-857D. Adjusting the whip was no help. I will look at changing the mast for a taller section later, but first I might, just for fun, rejig it to a design frequency of 21MHz and see how it does.

The license application for the St Mary's church activation is filled in and ready to post tomorrow. Ive spent some of today checking coax patch leads to see what I have and ensuring they are all marked with their length, this is invaluable in the field as knowing which coil is the right length for the job saves a lot of hassle! I have three lengths of RG-213/214 salvaged from work which need testing. One has N-types on it already, that just needs a check over to make sure the sheath is in good order, and a quick check of losses to make sure the conductors are sound. The other two, one of which is 50ft long, are unterminated at present and also rather dirty!

Im also just 1m short of 300 ohm ladder line for the ZS6BKW! So i'll have to order a pack of that, and the minimum is 5m!

Yesterday evening, I installed eight ground radials for the flag pole antenna. These are in slots cut in the turf of the lawn and then stomped shut. Actually, Tom did much of the stomping! Most of the radials go in a reasonably straight line, but the three to the east didnt have enough space. Two curve somewhat to fit them in, whilst one actually has to zigzag a bit!

I now have to find time to analyze the flagpole antenna to find its resonant frequency, which im expecting to be a bit below 20m, and then work out the necessary loading and matching.

Saturday, 14 June 2014

CHOTA antenna - ZS6BKW on Larkspur/Clansman Masts

 With the Churches On The Air day just three months away, planning is becoming more crucial, and nothing is more crucial to an amateur radio special event station than its antennas! Using the Clansman antenna systems, and the Larkspur mast, I have the ability to get on air, on at least one band, even if all else fails. But, ideally I want some frequency agility. I expect most operation to be on 40m, but the ability to use other bands will help if theres a SID, or just to bag a bit of DX if it goes quiet.

The plan is to use a ZS6BKW antenna, in a shallow inverted V configuration. The center will be held aloft at 7m by the Larkspur telescopic mast, with the ladder line feeder running down at an angle to the vertical along one of the guy lines (this keeps it clear of the steel mast). The two 5.4m Clansman masts will then hold the ends of the antenna wires. The antenna wires will be just over 14m of Clansman antenna wire each, run off the spool, with the spools own 40m long throwing lines used to attach it to the 5.4m masts. These throwing lines are so long, that by feeding them down the masts through the holes provided in the guying plates, as the mast manual says, means I can put the two 5.4m masts up in position, then put up the Larkspur mast, and with a little slack in the lines, then haul the dipole center and feedline up the mast using its own halyard. A quick bit of taking up the slack and tying off and were done.

The RT-11 Remote Auto-ATU will then be used, if necessary, with a short RG-214 patch lead from the antenna feeder, and then a longer RG-214 cable from the ATU to the operating position inside the church. Hopefully an on site prior test of the antenna can be arranged (at least one test rigging needs to be done somewhere!), and during this the MFJ ATU will be tried as well. If the manual ATU at the operating position shows no detriment compared to the remote ATU, then the manual one will be used on the day, as that means one less cable to route out the door!

If I can arrange a WSPR beacon, this will operate on 10m using the 10m fibreglass fishing pole mast and the 10m J-pole antenna. Theres little change that the Wispy beacon will be completed, but I could always do this with my FT-857D.

Anyway, time for a bit of blatant plagiarism! Below is a copy of Brian ZS6BKWs original antenna article. Ive done this to save me typing the details of the antenna design, and im sure Brian won't mind too much!

The July 1958 edition of the RSGB Bulletin contained an article by Louis Varney G5RV on a novel multiband dipole which did not require traps. Figure 1 (below) shows the antenna, later to become known universally as the ‘G5RV’.


Like so many good ideas it is so simple. It works as follows: On 20m the flat-top is three halfwaves long. Its feedpoint impedance is therefore low and because the open-wire line is one half-wave length on that band it merely transfers that low impedance to its other end and there presents a reasonable match to the ‘Twin’ feeder on the rig. On 40m the feedpoint impedance is very high (and inductive) because the antenna is now three quarters of a wavelength, but the transmission line transformer is now one quarter-wavelength and so functions as a quarter-wave transformer. Hence the high value of the load impedance, Z(L), is transformed into a much lower value, Z(IN), by the well-known relationship for the quarter-wave transformer:


Where Z(O) is the characteristic impedance of the open-wire line, typically 300-600 Ohms. It is rather like an automatic ATU hanging off the antenna! On 15m and 10m the antenna/feedline combination were again said to combine to present reasonable impedances to the twin-feeder, which was all that a valve power amplifier with link or pi-coupling ever requires. The tubes of that era coped far better with mismatches than do the solid-state devices of today!

On reflection, it soon became apparent that one should be able to improve on the performance of this antenna by using a computer to optimise the length of the flat-top and matching transformer such that the impedance presented at the transmitter end of the line more closely matches the 50 Ohms, plus or minus a few, that our modern finals will tolerate.

To do this we need to know the feedpoint impedance of a centre-fed dipole antenna as we change its length and as we change the frequency. This can be calculated but it by no means an easy task and a far simpler approach (and one that is probably more reliable) is to use the data which is available in the professional literature. Professor R W P King at Harvard University had fortunately provided us with this information in tabular form. To use it requires only that it be stored in a ‘look-up’ table in the computer. Given the frequency and the length of the antenna, we then have its impedance.

The next step was to consider the role of the transmission line transformer. How long should it really be and is one value of Z(O) better than another? Without going into any detail here suffice it to say that Louis Varney’s statement, way back in 1958, that Z(O) was not too critical is in fact not far off the mark. It has been shown that there is a broad peak of Z(O) values from about 275 Ohms to 400 Ohms, which will work adequately. This means that either homemade open-wire line or commercial 300 Ohm tale could be used. Do choose the best quality 300 Ohm tape though because that sold for FM-band folded dipoles doesn’t really weather at all well.

To determine the length of the matching section we use the standard transmission line equation which gives us Z(IN) if we know Z(L) and Z(O), the frequency and the length of that line. By re-arranging the equation we can find the length at any given frequency and Z(O) once we’ve used the ‘look up’ table to find Z(L). Of course Z(IN) is fixed by the required standing wave ratio on the 50 Ohm cable to the rig. Usually this VSWR may not be more than 2:1, and is always specified by the transceiver manufacturer.

Armed with this information writing the computer program is a fairly conventional procedure and will not be described here. Ideally a single antenna should operate on all the HF bands from 160 through to 10m. That is a tall order though so we would probably settle for a compromise of say five of the nine bands (including the three new ones.) Having chosen that number we then instruct the computer to change the flat-top length, the length of the matching section and Z(O) until it finds that combination of the three parameters which yield better than 2:1 VSWR on at least five bands. Clearly this involved an iterative or ‘going around the loop’ procedure and can take a fair amount of computer time, but the results are worth the effort.


In Figure 2 (above) are the details of the improved, computer-designed, G5RV. You will notice that the flat-top is shorter than Varney’s and that the matching section is longer. A velocity factor of 0.85 was used for the 300 Ohm tape. Particularly important is the fact that this new antenna is designed for use with 50 Ohm cable and not the 70-100 Ohm twin of 1958. No balun is specified simply because neither the theory nor considerable experimentation justified the inclusion of one. Simply interconnect the 300 Ohm tape and the 50 Ohm coax, taking the normal precautions to keep moisture out.

Both in theory, from the computer predictions, and in practise, when erected horizontally at a height of 13m, the antenna provides an acceptable match on the 7, 14, 18, 24 and 28 MHz bands. The original G5RV was tested by way of comparison (both with the computer and in the field) and found to be far less effective. Only the 14 and 24 MHz bands produced standing wave ratios of less than 2:1 when fed with 75 Ohm cable, as designed.

The new antenna was also tested in the very popular inverted-V configuration and the results showed, not unexpectedly, that the frequency on each band at which the best match occurred were all shifted somewhat lower, but the same general characteristics as discussed above for the horizontal configuration still applied. Likewise, changing the height above ground from 7 to 13m did not markedly change the situation either. It must be realised, of course, that the old dictum ‘the higher the better’ always applies.

Modern technology has been put to work to optimise an antenna conceived empirically nearly 30 years ago and the results should give the old G5RV a new lease of life.

Brian Austin, ZS6BKW

Now, I did intend making some notes here about 'Selby Shorty' and the 20m flagpole stealth vertical, but this has become rather a long and wordy post, so i'll do those on a fresh post instead!

Thursday, 12 June 2014

Clansman antenna kit

Much of my Special Event and field antenna kit is ex-MoD 'Clansman' equipment. After all, why go to all the trouble of designing stuff yourself when MEL and Racal have already done it?

I have a Clansman dipole kit, which consists of a dipole center (two spring loaded terminals and a BNC), and two very long lengths of kevlar braided wire and long lengths of line on a spool. These antenna wires are very versatile, and also have coloured cable ties on them at intervals of 1m. A single tie indicates a 1m mark, a double tie is used at 5m, and three ties 10m. So its possible to wind out the length of wire needed day and night.

I have a 27ft (8m) Larkspur push up mast. This is copper coated steel, and can in itself be used as a vertical antenna, as it has a base insulator! Even better, the clamp at the top allows addition of whip elements, allowing the mast antenna to be extended. I have a 2.4m collapsible whip, intended for the PRC320 HF manpack radio, which will fit.

I also have two 5.4m Clansman sectional fibreglass masts, one modified with securing bolts to allow the turning of a lightweight VHF beam antenna. These can also be used as vertical antennas, as each has a length of wire its length that attaches to the top section. The top section has a hole, into which can go - you guessed it - the 2.4m whip!

All this makes for a very versatile antenna system. The only thing I dont have is the 'counterpoise' kit, which is four 5m wires on a spool, but these sell for around £30 now, due I think to the popularity of re-enactment groups, or more likely the airsofters! But I can easily make one of my own for much less money!

And the reason for this write up?

I wasnt sure that the 2.4m whip would work 'as is' on the 5.4m masts, as the manuals say to use the adapter. So, I put it into the end of a mast today, metered the connection out, and agitated the coupling. The whip does indeed work like this! Adapter not required.

The 'Selby Shorty' Experimental HF Mobile Antenna

Ive been operating HF mobile for some time, using 'hamstick' type antennas. These are ok, but not the most efficient, or the smallest!

Now, generally, small isnt good at HF. And this is especially true when mobile. We want the biggest antenna we can safely use. But, invariably were talking loaded whips. If we can make the loading coil as efficient as possible, in other words nice and wide, airspaced turns, etc, then we can go some way to being more effective.

The ultimate goal of this experiment is to create a multiband HF mobile antenna, which will utilize an air spaced center loading coil, and a 'flying lead' system to select from tapping points on the coil to tune the antenna to different bands.

The first problem, is the air spaced loading coil. Such coils are commercially available, but unless your surname is Rockerfeller your unlikely to be in the market for them! I certainly cant afford the ones ive seen on sale! So, Its a case of, literally, rolling your own.

Some time ago, I constructed a former for just this task. Made from a section of 40mm diameter waste pipe, slit down its length, and with a length of hardwood strip inserted to 'open up' the slit to the original pipe diameter. Removal of the hardwood strip causes the former to contract slightly, allowing the coil to be slid off. This is all well and good, but of no use unless we can keep the coil in its form as it comes off! The trick here is to use some sort of spacer - in this case, lengths of 'flexible grommet strip' and liberal helpings of hot-melt glue.

The photos here show the construction of a smaller, thicker coil, using bare copper wire (ex mains flex) but demonstrate the principle by which the bigger coil was made.

Firstly, a strip of masking tape was wound onto the former, sticky side out. To this were stuck the cut lengths of grommet, with the slots facing outwards, equally around the former. I used three, but two or four could be used depending on application. The end of the wire to be wound is attached and secured by a bit of tape, and carefully wound around the former, each time resting into the slots on the grommet strips.

 Once all the wire is on (leaving sufficient 'tails' if required), a hot-melt glue gun is used to fill the grommet strip with glue. This is best done slowly and carefully, as the glue must completely fill the void and flow around the wire turns. Once the glue has set (ideally overnight!), the hardwood strip is pushed out of the former, allowing it to collapse slightly. The tape holding the tails is removed, and the coil slid from the former. If required, any bends, kinks etc in the wire should be straightened out first.

 With the former removed, the coil will look like that bellow. Push the masking tape gently inwards to separate and remove it.

And what you should be left with is a reasonably tidy, but at least functional air wound coil. The thicker the wire the sturdier it will be. The one below is for another antenna experiment.

So, anyway, onto the antenna! I had already made a 'mast' section from the cut down remains of an old hamstick. In order to put the metal ends on, a pair of hefty pliers/mold grips, a vice, and a blow lamp are needed! This one is particularly short, a mere 38cm long! I dont particularly expect good results from such a short mast section, but its enough to prove the concept.

The idea for this is in no way original. It is simply a variation on the well proven 'bug-catcher' design.
One problem with connecting a home made coil like this, is that in order to solder to the ends of the mast, the iron required is one from that mythical ham radio supplier that trades under the name 'B.F.O'! Not having a BFO soldering iron, I had to make use of the fact that previously this antenna section had been used for another trial, and already had enamel covered wire attached. Unfortunately, this wire, and that used to make the coil shown below, came form the same antenna supplier, who for reasons of saving a shekel somewhere used non-self fluxing enamel. This meant I had to laboriously and carefully scrape the enamel off with a knife.

The problem now was soldering, without melting the glue and causing the coil to unwind! This is easily solved by using a heatsink. In my case, a pair of locking forceps were clamped to the wire between the joint and the coil former! The wire tails of the coil were bent inwards and wrapped around the mast fibreglass section before being soldered. The solder joining the loop of each tail to itself and its connection to the antenna ends.

Not shown of course is the whip, just over 1m in length, that attaches to the top! All that remains now is to mount the antenna on the car, and run the analyzer over it. I have not really made much effort to get the number of turns right for any particular band, but am hoping it might be in the ballpark of 20m. Once ive analyzed it and know its resonant frequency, the coil can be reduced to make it work on a ham band.

It is purely an experiment though, I dont intend using such a short mast design mobile on 20m! Its more to prove the fact that I can manufacture my own, effective, air wound antenna coils. The fact that the enamel is hard to get off also makes this coil less suitable for the multiband design, where bare copper is preferable. That said, if it works ok on, say 15m or 12m, or 10m perhaps, then it might find its way into my mobile radio armoury!

Sunday, 8 June 2014

Getting on the web...

Some weeks ago I bought a mounting kit from Amtools to build a Cobweb antenna. After a couple of weeks wait whilst I treated myself to an automatic center punch from the far east, I took advantage of the good weather today to spend time in a dimly lit workshop drilling holes in aluminium plate!

The photo above is of the completed mounting plate from the underside. The one below is of the top side, where the spreaders will be fitted

I now need time to add the spreaders and the antenna elements. This is likely to be some weeks, which gives me time to get the balun made!

I also now have roof bars on my car, after 'repurposing'  the ones bought for Julies Ford Focus which she no longer has. One convenient aspect of this is that I can use the car as a mast base. But, before I can, I need to build a bracket that will allow me to clamp the mast to the roof bars.

As it happens, I discovered I actually have quite a few more U clamps than I realised! Two of the smaller ones will nicely clamp the bracket to the roof bar. A couple more larger ones are needed, plus some suitable box section aluminium tube, to make the bracket and mast clamp.

I also installed a 4ft ground rod underneath the 20m vertical (a.k.a Sams flagpole!), and finally got around to testing the antenna with the MFJ-259 analyser. It does seem that ive unfortunately cut it a tad short, and its resonant around 13MHz. No big problem, a couple of turns at the base for a loading coil will correct that. But, at 20m, and with just the ground rod, the VSWR is about 3.5:1, well within range of the remote auto-ATU I intended using with it!

So, what about actual radio? Well, today has been very warm, in fact too hot for me! So, in between a bit of gardening, antenna work etc, and before and after karate, ive had a potter about on air. Being the D-Day weekend and also Museums on the Air, theres plenty of special event stations about! Ive time for a few more before bed, and at last am enjoying the LED shack desk lighting I installed!