DF Antenna & Variable Offset Attenuator

Using this 3 element Yagi along with a modern 2m receiver, signals from a FOX station can be detected up to 20 miles away in open country. As far as the receiver is concerned, an S-meter is not essential, with the aid of an adjustable attenuator, you can use the signal to noise ratio of the monitored transmission to establish the relative strength of the signal.

Building an effective adjustable attenuator at 144MHz is not easy and requires a good knowledge of screening and coupling techniques. This is where the active attenuator scores, as virtually no screening is needed; mine only uses a tin enclosure because it was available and I could solder the 50 ohm sockets directly to the chassis, providing good continuity between both.

The active attenuator presented, is really only a simple mixer, combining a relatively low frequency local oscillator, in this case generated by a clock oscillator module running at 4MHz. The level of the 4MHz local oscillator signal is easily controlled using a simple potentiometer, rather like a volume control in an audio circuit.

You may be thinking at this point, why not just use a potentiometer to attenuate the FOX signal directly? Well it doesn't really work at frequencies above HF, it's down to screening again, there is too much coupling and bypass at VHF of the variable resistor. In addition the feed impedance to the antenna will be very poor causing its directional properties to be compromised.

Mixer

The mixing in this design is performed by a single silicon diode, it takes the incoming FOX signal, for instance 144.625MHz, via the beam antenna and combines it with the 4MHz local oscillator drive. This produces mixer products and there will be two close-in images of the FOX transmission, one at 148.625MHz and the other at 140.625MHz – these are the FOX frequency plus and minus 4MHz!


If we tune the receiver to either of these mixer products they are far enough away from 144.625MHz not to swamp the receiver when close to the FOX transmitter. In addition if we reduce the level of the low frequency local oscillator, we also proportionally reduce the mixer product we are monitoring. It's a simple idea and was first aired by the ARRL back in the late 1970s, The design works remarkably well and produces a maximum attenuation range of zero to in excess of -60dB. Which with a small rubber ducky antenna will enable you to get within touching distance of the FOX transmitter, while the Yagi is used for initial cross country bearings further out.


The design shown in Figures 1 & 2 is not mine and was borrowed from Joe Moell - KØOV, there are many other derivations of the design, some using discrete low frequency crystal oscillators which are available as mail order kits.

 

Figure 1. Complete mixer circuit, the grey line around the circuit is the enclosure.

Figure 2. Internal layout of my attenuator in a small tobacco tin.

Antenna

The 3 element Yagi is constructed from a budget 5m steel measuring tape, plastic electrical conduit and fittings from Screwfix. It cost less than £15 to build, excluding some scrap coaxial cable, self-tapping screws, cable ties and Velcro patches. Again I claim no originality for the the antenna design shown in Figure 3 & 4, being sourced from Jeffrey Bail - NT1K web pages, mechanically it's been modified to work with the cheapest fittings I could get. 

Figure 3. Completed antenna with elements folded back on themselves, it can be used in this configuration when thrashing through hedges causing less element tangle.

A hair-pin match is used to balance the antenna feed to the coaxial cable, which is then coiled around the boom to form an RF choke, helping preserve the antennas directivity with reference to feeder position.

Figure 4. Hair-pin match and built in RF choke.

The antenna has a good front to back ratio and can be used back to front in order to provide more attenuation when approaching the FOX station.  Please follow the homingin.com link below, as it's a mine of useful information on fox hunting techniques. Radio fox hunting is great fun and can be done on foot, in a local park or wood,  or by car, although fuel being at a premium this is a less attractive proposition. It’s always nice to adjourn to a local hostelry when the FOX has been found, for refreshment and a wash-up meeting to compare notes. 

73 Rick G6AKG

References:

Text and artwork © 1998-2019 by Joseph D. Moell - KØOV. All Rights Reserved. http://www.homingin.com/index.html#toc

3 ELE. TAPE MEASURE YAGI, Prints By: Jeffrey Bail - NT1K. All Rights Reserved. 

https://nt1k.com/wp-content/uploads/2012/11/3L-Tape-Measure-Yagi-Assy.pdf

Linux for the Masses

So after 10 years of messing with the Linux operating system (OS) I've taken the plunge and ditched Windows. This period has seen the launch and displacement of Windows 10, with the last straw being the most recent upgrade to 11, which calls for the very latest tech to support it. I'm not in the habit of throwing out perfectly good hardware to justify the whims of a broken consumerist system, so I've pressed my ageing hardware into another stint of operation.

I've kept one really old machine which runs DOS and XP for re-programming ex-taxi radios for amateur use, which  require very slow serial port communications. The rest have been upgraded to Linux, two flavours have over the years caught my eye, both of which are serviced by the Debian Linux stable .

My daily driver is a 64bit Fujitsu Desktop with 4GB of RAM and runs Zorin Linux, I've setup the machine to look and feel like Windows 10, so as not to confuse my wife, who's knowledge of the computer doesn't run beyond using the browser and word processor.

Desktop of Zorin Linux

My hobby machine also runs Zorin Linux, the Lite version, now running on 64bit i5 Core Toshiba Laptop with 4GB of RAM, this has adequate grunt to handle my amateur radio software, including those written for Windows via Wine.

One interesting thing to note is the support for 32bit machines is disappearing fast and in order to keep a laptop running at work for browsing outside our secure system, I've opted for Sparky Linux, which again is Debian based. This has a very light footprint and runs easily with 2GB of RAM.  

Desktop of Sparky Linux

Overall I'm very pleased with Zorin Linux, I'm not a programmer and very rarely resort to running commands in text. Both flavours of Linux mentioned are written for normal folk with a modicum of intelligence. The app. store for Zorin is very shiny and easy to use and offers the biggest selection of software, the same as offered to those who use Ubuntu Linux. Support for both OS are excellent as are their user forums.  

All open source systems require external funding to keep the lights on and the bills payed. So if you find yourself switching to either above and your happy with the experience, please look to make a donation for their continued development and upkeep.  

73 Rick

Filling the void

When the Bournemouth repeater GB3SC closed down the 2m band turned into something of a local backwater. Jon G7ICH, myself and others used to hold court every morning on our daily commute via SC. To say we missed it would be an understatement, but life moves on and we let it go, because people changed jobs and their priorities change; the stuff of life!

However, with the coming of the digital revolution including Fusion and DMR, it was considered that a local DMR repeater might be a nice thing to have, particularly when connected to Brand Meister. I found a site on a high point overlooking the Poole conurbation which proved to be a good spot for our 70cm box GB7DR. The repeater deck was provided by my very generous employer and fellow ham Nick Foot G4WHO (now silent key) and the infrastructure was paid for by the Poole Amateur Radio Society. The site is provided free by Alfatronix Ltd (https://www.alfatronix.com) who keep the repeater fed and watered with a smattering of internet access. Now the bugs in the intranet firewall at the site have been sorted by Jon, the repeater gives trouble free access to DMR system - give it a go.

Analogue

I'm a self-confessed analogue amateur/engineer, and I've never liked the speech quality provided by the DMR system, plus being digital you can never have a marginal contact, when the signal gets too weak it just stops trying!
Jon, myself and several other local amateurs thought we should plough on and aim to get an analogue repeater to replace GB3SC. After asking around we managed to snag a free 2m deck in the shape of a Philips/Simoco PF10. This was kindly donated by Andrew Barret G8DOR, (past) ETCC Member for the South West. We then became doubly blessed by a chance meetings with an ex-radio amateur, turned PMR engineer Gary J. Simpson (www.commsouthern.co.uk) and ex-Southern Gas communication engineer, Mark G4SDR.

Suitable Site 

At this point we decided to look for a suitable site - having lived in the Poole/Parkstone area all my life I know the topography quite well, and the location of most of the PMR repeater sites since becoming a radio amateur in the 1980s. Several locations were selected and letters were written to site owners, only one came back as a maybe and that was from a social club in Upper Parkstone - just spitting distance from a PMR site that was about to be decommissioned. We had a couple of meetings and the senior trusty of the site, a once enthusiastic SWL, found us some attic space and a outside wall to mount the antenna, roof space proved to be warm and dry and had easy access to authorised personnel.

Lock Down

The repeater ran for nearly a year, but due to lock-down rules our friendly social club didn't ride out the economic tsunami caused by the virus. So we had to remove the repeater and start again at a new site. We got permission from Nick Foot my managing director, just weeks before his demise, to set the repeater up on a old PMR mast placed on his industrial unit by a previous tenant. The repeater was given space on a storage platform in the unit and given an AC supply and access to WIFI. The site was not as well served by the local topography, but it proved adequate after the application of an low noise receive preamplifier, which then made it accessible to most mobile stations in and around the Bournemouth and Poole area. Thanks go to Alan G7NYI for providing a brand new antenna, as the previous item had proved unreliable, and to Carlo G7IRU our group antenna rigger for fettling the feeder and mast instillation.

Something Borrowed.....

Since the first instillation of the repeater, we had been given two scrap Simoco TF2000 decks by Gary Simpson, both needed repair but after some TLC from me and help from the guys at the Simoco UK service depot both were made operational. We had by the second instillation acquired support and equipment from: - Les, G0FAJ, for the antenna and bandpass filter, Paul Marsh M0EYT, Jules Smith G0NZO and Kevin Nice G7TZC for the Heliax feeder and filter cavities. As the four cavities didn't offer the required isolation needed for the industrial location, John Fell G0API provided a circulator from the original GB3SC repeater and a low-noise pre-amp. In addition we had some financial support from the membership of the Poole Amateur Radio Society G4PRS, and a good deal of technical expertise was also provided by M0EYT, G0NZO and G0API.

A Repeater Is Borne 

We were asked by the Poole Amateur Radio Society G4PRS, if we could christen the repeater with letters to denote it was the Poole repeater, the closest we could get was GB3PB - Poole Bay. So the GB3PB repeater is borne a second time, rising from the ashes of its previous existence, providing an analogue gateway linked via Allstar to the internet. However, due to security issues it was not possible to access the repeater directly from the internet, so using RF activations and the DTMF protocol described on QRZ.com entry for GB3PB, the Allstar system can be activated. In order to offer this facility, Jon G7ICH (who is now recognised as the IT expert in the group) used a PC based application to control the repeater and a Raspberry PIE to interface with the Allstar system.

Repeater Community 

Unfortunately, COVID saw the demise of my friend and long term employer Nick Foot G4WHO. Nick was a leading light in the development of amateur repeaters locally (GB3SC & GB3SZ both silent key) but his legacy lives' on in GB7DR which he purchased, and the siting of GB3PB at his old company near Wimborne. In fact without the support of all the above persons and many I have not named, GB3PB and GB7DR would not exist. The hardware of both systems is owned by the amateurs that provided it and is a testament to the term community repeater. The best complement you could pay to the memory of Nick and co, is to use our repeater and enjoy the hobby of amateur radio and mobile operating.

Repeater Details

GB7DR occupies Channel DVU34, RX-439.425MHz, TX-430.425MHz, Colour Code 5, Slot 1 - Talk Group 9 (Local QSO), Slot 2 - Talk Group 950 (Brand Meister). Located Canford Heath, Poole. Keeper is Jon G7ICH. For complete operating details see the repeater entry on the QRZ.com web pages.


GB3PB is a 2m analogue Allstar enabled repeater serving the Poole Bay conurbation and outlying areas, AllStar node 510620. Channel RV50, TX-145.625MHz, RX-145.025MHz RX, CTCSS 71.9Hz. Located in Ferndown. Keeper Jon G7ICH. For complete operating details see the repeater entry on the QRZ.com web pages.

Last go round

The repeater GB3PB gives good coverage over the Poole and Bournemouth basin with outstanding coverage to the north of Poole, despite for the height of the antenna. It does however, have a few holes in its coverage due to the nature of the local terrain. If we could get the antenna another 5 to 10m higher, then most of the issue would disappear, but the antenna is as high as our very generous landlord would permit. :)

QRP VSWR meter that actually works.

I've been searching for a simple and cheap VSWR meter circuit that doesn't use a resistive bridge. The resistive type works well but I've found them a nuisance, as they have to be switched out of circuit when not in use as they attenuate both RX and TX signals.

Just by chance I was doing a Google image search on VSWR circuits when I came across the following from PY2OHH. This design is not easy to find on the web so I thought I would share it with you here.  In most cases I only build the reveres power side of the circuit as this is the parameter I'm normally interested in. I can assure you it works very well indeed, even with a 1mA FSD meter.  However, It does tend only runs out of puff on Top Band where it requires the full 5W to get a meter indication, all the other bands up to ten meters works fine down to 1W.

I plumped for the coiled strip-line version and used a germanium point contact diode, similar to a OA81 device to get the best sensitivity. The design is simplicity itself with no complicated coupling transformers, plus its really easy to align.

The link below takes you to an English version of the design. Miguel is experiencing some issues with his web material so I've copied his design to my Google drive.

PY20HH Design on my Google Drive

Picture from PY2OHH project notes.

30MHz and Above.

Why is there a demise in analogue amateur activity on the VHF and UHF bands, digital modes such as WSPR, FT8, Fusion and DMR are not completely to blame. The release of  digital modes, to Foundation class licenses has, in my mind, caused the problem. If a 24 month period of analogue operation on bands above 30 MHz was included, with QSL proof, became a stepping stone to the next license class it would help. Exposure to the 6m, 4m and 2m band SSB and FM would have made these operators aware of the DX possibilities of these bands, and given them a life long appreciation VHF and above. As it did me as a Class B operator under the old license scheme. I often still sit on hilltops listening for summer tropo signals to appear on hot days and for Sporadic-E openings just before a good Autumn thunderstorm. They have missed all these mystical and magical phenomenon which are a delight to me even after all these years.

Eazy Hi-Z Phones

I was looking at throwing out some scrap office telephones at work but we are not able to dump electronic waste in to our refuse. So I stripped them down to recover the plastics and was left with a pile of electronics. All good stuff , Jolly Useful Necessary Kit (JUNK) - switches, electret microphone inserts, medium impedance earpieces and other goodies like piezo sounders. Seeing the sounders took me back to an article I saw a couple of years ago where an enterprising SWL had fashioned a couple of these sounders into a pair of high impedance headphones.

Now from time to time I like to build a crystal sets just to remind myself why I got interested in radio, trouble is the only devices I have for listening on is one of those uncomfortable crystal earpieces, salvaged from an old hearing aid. So I decided to build a pair of high impedance phones using a cheap pair of ear-defenders and two of the piezo sounders.

The sounders have a very high impedance, too high for normal use, so I wired a 43k Ohm resistor in parallel with each sounder and then placed a sounder each defender ear-cup. I wired them with the cable salvaged from one of the phones in the same configuration as a stereo headset, terminated in a miniature stereo jack plug. This enables the inserts to be wired in series or parallel, in parallel the impedance of 21k Ohms is just about right for crystal set use.

Any sounder can be used, from the type you may find in musical greetings cards to the old trim-phones - bigger the sounder the better the low frequency response.

I'm very pleased with the result, my new headphones are comfortable and have a nice frequency response for communication use, together with a couple of useful resonances for peaking Morse signals - particularly good when used in conjunction with one of my simple valve regen receivers.

Selection of salvaged piezo sounders

Piezo sounders installed in cup including 

original sound deadening foam behind each insert

Finished phones including salvaged curly wire

 

 

Link to original article

https://drive.google.com/file/d/1hubS5vzhiqMuaMFrqDx2c5GLJDHal8lT/view?usp=share_link

Modified End fed matcher

Since my original post regarding my linked end-fed half-wave antenna and matcher I've decided to extend its range to include 80m. The switch I fitted to add the 60m band was replaced with a centre off change-over type. The extra place on the switch puts a 600pF silver mica cap in parallel with the original tank circuit - 600pF is not a common value so I made it up with several devices in parallel. So with the new switch in the central position the matcher covers 40m  to 15m, first position on the switch enable the 60m band and the second the 80m. The test button that was fitted to put a 3.3k termination in place of the antenna was replaced with a toggle change over type as the original was too hard to operate with cold fingers. The 1k resistor in series with the VSWR dip indicator LED has been replaced with a 4.7k to improve the dip indication - see the accompanying circuit diagram.