Inverted L

I have been lacking the ability to operate on the lower HF bands, while my small ‘multi-band’ OCFD could be used on 40m trying to use it on 80m was nigh on impossible with my ATU. As you would expect even if matched for a useable VSWR the actual performance has been compromised.

I needed a cheap and relatively unobtrusive solution and I found one in Len Paget’s (GM0ONX) design for an inverted L. The full details were printed in the Practical Wireless magazine several years ago and PDFs are available for download from Len’s website. The designs make use of coaxial traps, one for the 80m version, two if you want to add a top-band (160m) option.

Opting for the smaller 80m version I set about building one. Using an old fibre-glass fishing pole about 5m high at the far end of the garden tucked behind the summer house which I could collapse down and then retract the wire elements when not in use so hiding it from view.

The fun and interesting part was building the trap. They are formed by coiling some coax, in this case RG58, round a former such as plastic waste pipe. I had a bit of scrap pipe but it was 32mm not the 40mm diameter type described in the article, thinking it couldn’t make ‘that much difference’ I built one using the same number of turns but the resultant antenna wasn’t anywhere near resonant according to the analyser.

I decided I needed to test the trap’s frequency response. I found a YouTube video by Dave Tadlock (KG0ZZ) where he demonstrates using a MFJ ‘Grid Dip Meter’ adapter on an MFJ antenna analyser to test coaxial traps.

The adapter consists of nothing more than a coil of wire on a suitable former so I made my own to use with my AW07A analyser. It worked a treat and I discovered my trap was way off frequency. In the mean time I had located a useful PDF document by David Reynolds (G3ZPF) which informed me I actually needed 180cm of RG58 round a 32mm pipe to make a suitable 7MHz trap, so I made another.

I made a small video showing the traps and how I tested them.

The resulting antenna seems to perform well, but it does sag a little due to the weight of the trap and wire and the flimsiness of the pole. But I have made a number of contacts on it and used it during the RSGB 80m CC Datamode contest a few weeks back. It was my first go at this contest but once I got the hang of operating it was great fun.

Feature Tech AW07A Antenna Analyser – First impressions

Christmas seems such a long time ago and one of my presents was a Feature Tech AW07A HF-VHF-UHF Antenna Analyser which I have finally been able to try it out.

It is about the size of a thick paperback book and is a powder coated steel case similar in style to that used by MFJ equipment, indeed the MFJ-266 analyser appears to be a re-badged version albeit for a lot more money than this unit can be purchased.

It can be powered by batteries fitted internally or by an external supply and is supplied with a power cable for connection to an external supply, mine was white/black rather than the normal red/black cable. It has a N-Type socket for the antenna and comes with two adapters for PL259 and BNC connectors.

It has a power button near the external power socket, two buttons on the top select HF and VHF/UHF operation and two other buttons marked UP and DOWN to select operating mode and/or the frequency band being used. Unfortunately one thing it doesn’t come with is a manual but a copy can be downloaded from QSL.net or a slightly different version from the manufacturers website. But I actually downloaded the manual for the MFJ-226 has it is much more detailed.

The front panel decal and manual state it can be run from 10.8-12V, in fact the manual states it should ideally be less than 12.5V and no more than 13V. While doing some research I found the reason for this limitation hidden away on this aliexpress webpage “Avoid higher than 13V power supply circuit for the UV segment may be damaged due to excessive power dissipation.” So this would seem to rule out using a standard 13.8V power supply.

It can be fitted internally with eight AA batteries and this is the way most people would use as it offers portability. Removing four screws allows access to the battery compartment and the internals electronics seem well built.

It takes eight AA batteries, in two boxes. The battery boxes have lids secured with a small screws and are fixed to the case using simple sticky pads, while secure at the moment I can imagine in time the adhesive could dry-out and become unstuck leaving the battery boxes loose inside the unit.

The display is a simple two line LCD with an optional bright back light which can be turned on during the power up sequence. The display shows the battery or supply voltage and pressing Down puts the unit into a frequency counter mode. Pressing Up puts into the antenna analyser mode.

In the analyser mode it is a simple case of selecting the HF, VHF or UHF mode. VHF works from 85-185MHz, UHF is 300-390MHz, the HF is split into six overlapping bands A: 1.5-2.7 MHz B: 2.5-4.8 MHz C: 4.6-9.6 MHz D: 8.5-18.7 MHz E: 17.3-39 MHz F: 33.7-71 MHz selected using the Up/Down buttons.

Turning the vernier  tuning knob adjusts the generated frequency the antenna is being tested against. I connected the analyser to my 2m YAGI antenna and turned the knob to find the lowest SWR

The manual describes what is being displayed (on UHF just the SWR is shown)

“139.763 MHz” is the frequency
“V “is the band (A,B,C,D,E,F in HF, V in VHF and U in UHF)

The bottom row shows the complex impedance Z = R + jX, so on this screen 

“41” represents R = 41 ohms the resistive component
“18:” represents the reactance component value, jX = 18 ohms
“45” is the overall complex impedance magnitude Z = 45 ohms
“1.5” is the SWR value

As you can see for a 2m antenna something isn’t quite right! The antennas were down due to last weeks strong winds so I was taking the opportunity to do some maintenance and tweaking of the 2m antenna since I’d seen an increase in the SWR during recent UKAC contests. I had suspected feeder issues, possible water ingress but I tried a dummy load at the antenna end but that read as expected (Z=50ohms) and metering the continuity of the feeder showed no issues, it just seemed to be resonant at too low a frequency.

The analyser confirmed what I’d observed with a normal SWR/Power meter a higher than desired SWR in the middle of the SSB section of the 2m band.

Unfortunately I was unable to get it any lower than 2.5 and most adjustments seemed to increase the SWR.  For peace of mind I double checked the analyser by swapping the feeder on to the 2m/70cm collinear and that was spot on

again I double checked the SWR readings back in the shack using the normal meter

While I try to sort out the antenna issue I can say the analyser seems to do its job well. The tuning knob is a little twitchy and has a bit of play which makes setting the frequency accurately a little harder than it should be but hopefully that might improve with use.

The unit also has other functions none of which I have used yet but it is bonus to have some useful test functions available in the shack.

The AW07A can be used as an inductance/capacitance meter by powering it up with the U or D button held down. The inductance or capacitance of a component fitted across the antenna socket is then displayed and this can be done for any test frequency by selecting the band and turning the tuning knob.

As I mentioned earlier the unit can also function as a frequency counter that can measure signals between 1 and 500 MHz and can be used to give an indication of relative RF field strength. A signal source or an external antenna that yields a usable signal level may be connected to the analyser’s antenna jack. The usable signal range is quoted as -20dBm (30mV)  to +10dBm (1V). Note that the display reading is a RMS value.

Obviously in the antenna analyser mode the output which is approximately 2V in magnitude can be used as a signal source, with 20dB of second harmonic suppression. 

The MFJ manual goes into some detail of how this all works and how to use the analyser for a number of common tasks such as checking baluns, making 1/4wave stubs or measuring velocity factor of coax.

While the AW07A has some obvious shortcomings and may not be a precision device I am impressed with it and what it can seemingly do. It is shame about the lack of a manual but I am not sure getting one is justification for the premium price of the near identical MFJ unit.

Antenna Update

Tuesday night is RSGB UKAC contest night, this week on 432MHz. Following the frustrations of the weekend I spent a therapeutic Monday evening cutting various lengths of RG213 and fitting N-Type connectors.

I have now got a coax run through the wall into the shack, with another to follow soon. To facilitate ‘switching’ between multiple aerials I have fitted each aerial with a length of coax running down the pole, terminating in an in-line N-type socket near the base. It is a simple case of connecting the appropriate shack coax, fitted with a n-type plug, to the appropriate socket.

To keep everything water and weatherproof I have opted for a DRi-BOX. These are inexpensive plastic boxes sold as waterproof housings for outdoor electrical installations such as garden or Christmas lights.

The lid has a silicon seal and there are a number of cable entry points with a flexible seal. When the lid is firmly clamped securely in place the box is effectivly watertight.

It is a bit of a fiddle with the thick RG213 but it seems to work well. There was a vicious thunderstorm and downpour yesterday afternoon and the Dribox lived up to its claims after sitting in a few inches of water.

Still awaiting the X-50 collinear on the top!

The pole now has the 2M Yagi and the 70cm 7-element ZL-Special fitted. The 70cm aerial is far from optimal but I was looking forward to giving a go with some decent coax.

Tuning around prior to the start of the contest and the band seemed quiet, hearing just a strong local operator. The contest start time passed and I was met with a wall of static only hearing the occasional very weak signal. I tried unplugging and reconnecting plugs, new patch lead, took the VSWR/Power meter out with no effect after 20 minutes I gave up. I decided something was obviously wrong with my new installation at the top of the pole.

I went back into the house where the wife was watching some dreadful house/diy/makeover program on TV which I could only manage for about 15 minutes. Grumpily I went upstairs and fired up the FUNCube Dongle and twitter and realised I’d forgotten and completely missed the first pass of the newly launched UKube-1, unlike some lucky ones. Idly I tuned to 70cm using the discone in the loft and could see a waterfall of signals! Including that local operator with a lot of splatter considering he was running just 10W

Going back into the shack and things had improved, so perhaps it wasn’t my setup! After missing nearly a hour I searched and pounced again, just making 14 contacts but achieved my highest score so far for a 432MHz UKAC contest, still operating as M6GTG in the low power section.

Various operators have commented on the weird/poor/flat conditions last night, so perhaps I shouldn’t have been so dismissive of my ability to put up a decent antenna!

The best laid schemes o’ Mice an’ Men

To quote Robert Burns

The best laid schemes o’ Mice an’ Men, Gang aft agley,
An’ lea’e us nought but grief an’ pain, For promis’d joy!

That sums up my weekend perfectly.

After becoming licensed my eagerness got the better of me and in a moment of weakness I’d purchased a couple of 10m ready made cables from a major supplier only to discover quite quickly they were made from low grade RG58 coax (you can literally count the number of strands in the braid) and the connectors while serviceable were so poorly fitted they fell off!

James that PL259 has fallen off again!

I had refitted the connectors using some solder this time and had managed to blag a 20m length of good quality RG58 (this actually has some braid) and I am ashamed to say these cables have been the weak link in my set up for far too long and needed to be upgraded.

I also needed to sort out the mounting for my aerials. Up till now I have been using a 5 meter telescopic painting pole that had cost around £16 from B&Q, it was okay when I was just clamping one aerial at a time to it but with the purchase of the rotator I has been chancing my luck with the loading, narrowly avoiding catastrophe when pushing the pole. I also couldn’t fully extend the top section as the tube and joint were potentially weak. It was guyed quite well but was far from aesthetically pleasing, even in the summer sunshine!

6M Moxon up on temporary pole

I couldn’t fit anything to outside of the house, not only for logistical reasons but also it wasn’t sanctioned by the station manager. Anything on the side of the house would be highly visible from the road and the back wall of the house has too many large windows and an extension making it inconvenient to fit and access anything.

Never fear I had a plan, a 20ft (6m) scaffold pole bolted using swivel joints on to to an another pole concreted in the ground in the back garden. The garden was extensively landscaped a few years back but it was before I became licensed, so I hadn’t planned ahead. With careful negotiations with the station manager I secured a location where I could put it.

Thanks to my local handy man I now have two 10ft scaffold poles, sunk to a depth of nearly 5ft and encased in concrete in the corner of the lawn. A few inches of soil was left on the top to allow the grass to grow. They have been left for over a fortnight to completely set.

Scaffold pole sunk in ground

I ordered a 100M reel of RG213 coax from Nevada Radio along with plenty of high quality N-Type connectors and various clamps and intended to sort out my antenna set up this weekend and banish the abysmal RG58 coax and PL259 connectors to some dark corner of the shack.

Before I took everything down on Saturday I managed to make contact with GB0TDF the special event station being run by the Denby Dale Radio Society from Cartworth Moor, Holmfirth for the Tour de France Grand Départ

A few months back I picked up a cheap rotator at the Dambusters Hamfest. It is designed for television aerials and isn’t heavy duty but I was sure it would cope turning with the small 2M, 70cm Yagi and a 6M antenna on the same pole, with the X-50 collinear on the very top. However I was concerned by the potential lateral loading.

The rotator is a generic design and I spotted that an optional support bearing is available as an accessory. I chanced on one via Ham Radio Deals and had salvaged several good lengths of galvanised pole from a skip where I work. So the plan arrangement was as shown.. simple right? 

Planned arrangement

It turned it a frustrating morning after cutting the metal pole to length, bolting and clamping everything together I tested it at ground level with no antennas and the rotator refused to turn correctly and I narrowly avoided burning it out.

I checked poles and they were true, the rotator was free running as was the bearing. I unbolted, fettled and just couldn’t make it work. I went away and had a beer while watching some of the Tour de France on the TV and in a moment of clarity realised what the issue was.

The problem was the diameter of the salvaged pole I was using. It was was slightly narrower (a couple of millimetres) than the hole in the bearing. I’d assumed it would line up with the rotator as it was similarly clamped, however when all clamped up top and bottom the pole wasn’t perfectly perpendicular and wouldn’t turn due to the eccentricity, Hopefully the drawing illustrates the problem.

Annoyed by this basic school-boy error I reverted to Plan-B for the short term, no support bearing! I was in bad mood now and so decided to leave the rewiring to another day. So I quickly put the 2M yagi on the existing coax as a test on the new scaffold pole to make a few contacts for the VHF NFD.

Temporary installation to test scaffold pole

Walking up the scaffold pole is straight forward, I have bolted a small cut off of scaffold across the top of support poles to act as a safety stop, lowering it likewise easy and I will certainly build up the muscles!

I managed to grab just 8 QSOs but was otherwise engaged for the rest of the weekend, however I was encouraged by the distances.

A few 2M QSOs during the NFD, Tour de France, Grand Prix and fixing a dripping tap took priority 😉 pic.twitter.com/BECaepRIlu
— Andrew Garratt (@nerdsville) July 6, 2014

I hope to get the 70cm antenna up tonight on the RG513 ready for the 432MHz UKAC on Tuesday evening.

The birds like the new setup

My seemingly magic HAB antenna

Since visiting the UKHAS Conference and getting my Foundation licence I have come of out of my lull with a new found buzz and have been busy with my radio gear.

I have helped track a few more High Altitude Balloon (HAB) flights, uploading the received telemetry data to the Habhub server. Visit the UKHAS website for more information on how to become involved.

I am still surprised by the performance of the loft antenna I am using to receive the HAB telemetry. It was constructed as an experimental wide band antenna solution to use with my scanners back in 2007. I discovered some basic plans on the internet for this simple home brew antenna (originally hosted on a Geocities website, but is thankfully archived on Reocities here)

It is a modified bicone design and is constructed from nothing more than a couple of metal coat hangers fixed to a piece of pvc water pipe, with a 10m length of RG58 coax as the feeder. I used it successfully as a portable solution, mainly for airband listening and stuck it on top of a fibreglass fishing/flag pole when camping away at dog agility shows as these photos show.

Eventually it ended up being mounted up in the loft, suspended on a piece of string from the rafters and had largely been unused since I acquired a discone a few years ago.

When I first started tracking the HAB payloads I naturally used the discone but soon became frustrated by its variable performance and so tried this antenna instead and was amazed. Something appears to be just right with this antenna on 434MHz when used together with my FUNCube Dongle PRO+

I haven’t analysed why is seems to have such a sweet-spot on these frequencies and I am not touching it, moving it or even taking another picture of it in case it loses it’s magic properties! Remember these HAB flights are only 10mW and this antenna is under a slate roof!

On Friday (20-Sept-2013) Adam Cudworth (@adamcudworth) launched HABE-10 which involved a normal tracker on the balloon due to burst at around 35km along with a secondary tracker payload of a 3D-printed man that was separated at approx 27km, the two payloads being tracked separately as they fell to Earth. During the accent the main tracker also transmitted SSDV images from an on-board camera. Unfortunately this failed during the mission but some pictures were received before it did, as you can see I managed to successfully receive this 300 baud RTTY signal from home and uploaded packets to help reconstruct the images. The original images are at http://ssdv.habhub.org/HABE/2013-09-20

 


 

Friday afternoon also saw the launch of Leo Bodnar’s B-13 pico floater. Leo has become something of a HAB superstar following his amazing B-11 and B-12 balloons which broke the duration record for an amateur balloons (as reported on the Southgate ARC website) flying over many countries before contact was lost.

Sunday saw a launch of MOD-1 by Ugi, which again I received rather well as can be seen on this pie chart.

Today saw Steve Randall (G8KHW) launch two of his XABEN flights, XABEN-56 and XABEN-57. The main transmitters were on 434.250MHz and 434.300MHz and I was able to use the multiple VFO option of SDR-Radio.com V2 to track both launches, and as you can see from these pie-charts I made number three in the tracking charts! I achieved this while using a VNC connection to control the receiving station from work, just checking occasionally to correct for any drift in the frequency of the signal caused by the temperature variation of the transmitters.


Antenna Mast

My current antennas are mounted up in the loft, the coax being dropped down into an upstairs room and connected to the scanners and receivers. So while I have moved a great deal of my junk into the shack I am stuck to just doing satellite work there for lack of a decent antenna.

I have purchased a dual band X50 2m/70cm antenna of eBay and plans are under way for the erection of an antenna mast/pole to mount it on!

What I have decided to do is purchase some galvanised scaffold pole, I will sink one around 6ft into the ground, which will be concreted in, giving a 6ft tall post. Then using some swivel clamps I can then clamps another longer pole to that.

By using some swivel clamps it means I can lower/remove the pole, allowing for easier maintenance and protection during high winds. By easily removing it I can also hopefully claim it is temporary and only raised when being used…

This is the sort of thing I plan on doing.

Skelton Transmitter Site

Just spent the last week in Cumbria and the Lake District on holiday,  for part of the time we were camped near the village of Skelton around 5 miles north west of Penrith. Skelton is famous for it’s huge transmitter site. The primary function is shortwave broadcasting, but it also supposed to be the site of a VLF array used for communication with submarines.

While out finding some geocaches I managed to take a few photos of the impressive antenna arrays.

 
 
 
 
 
 
 
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