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.

Operating /P for the 432MHz UKAC tonight

The 432MHz UKAC contest is usually a pretty dismal experience for me, low elevation, local noise and a mediocre antenna makes for a difficult evening. Last weekends VHF/UHF contest and some tests with fellow club member Stewart​ (M0SDM) on Sunday evening convinced me to try operating portable this week.

I have a small Moonraker 7-element ZL-Special on the main antenna mast, purchased originally for monitoring satellites and was pressed into service for SSB when I got licensed. It has never wowed me performance wise and I have been intending to replace it for quite a while but since I only use it one day a month it hasn’t been a priority. So the mast came down last night and I removed it so I can take it out with me to operate portable from some higher ground tonight.

Using the 2m delta beam in June 2014

Last year I had a go at operating portable from the car and posted a write-up. It is my intention to repeat this exercise but with the 70cm antenna on top of my ‘painters pole’ mast. I have serviced the antenna and fitted a new short run of quality coax and spent a far amount of time with the AW07A analyser adjusting the antenna’s tuning capacitor and have got the VSWR right down to 1.1:1 on 432.200MHz so things should be optimal.

I am looking forward to this evening, hoping it pays some dividends.

As I mentioned the 24 hour March 144/432MHz VHF Championship contest took place last weekend. I took part to give away some points just grabbing a few short sessions with the radio. I concentrated on the 2m band due to my issues on 70cm.

In my AW07A analyser review I mentioned some issues with my 2m LFA YAGI, thankfully these have been resolved. The use of some wire wool to remove some corrosion and a hacksaw to take 10mm from the long elements of the loop allowed the end elements to ‘trombone’ in sufficiently to get the antenna resonant and the VSWR is down to 1.2:1 on 144.300Mhz.

I only made 18 contacts, but was happy with the distances achieved with 30W, getting a lot further south than I normally do, given I am 18m ASL. There was also some local wideband noise (I captured a screenshot on the SDR) and the conditions gave some interesting fading.

M0NRD QSO map March 144 VHF

Noise across the band

   

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.