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.

Prepping for some more /p

Following the eggcitement of last week I am been taking it a bit easier this weekend preparing for some portable operation in Scotland.

I have made a linked dipole inspired by the ones sold by SOTAbeams, it is a simple inverted vee supported by a 9 meter fishing pole the wire elements are made of sections which are joined together depending on the band required.  I have made a two band version, 20m/40m and it tuned nicely using the analyser. 

As a backup I have also revisited the “Magitenna” which disappointed last time I tried to used it. Firstly I replaced the original wire which was too heavy for the low power I run and is very prone to kinking.

While doing this I discovered that the terminal post on “the special matching unit” was simply turning when I tried to tighten the wing nut. This pointed to a broken or at best poor internal connection and I was forced to open up plastic box to make a repair, sadly “forced” was the operative word.

One of the four screws was inexplicably super-glued in place and being cack-handed I soon ruined the head trying to remove it and had to drill it out. Once the screws were removed I then found the lid too was glued in place at several points and had to prise the lid off trying to minimise any damage in the process.

Given that nearly all other ham equipment I own is designed to be serviceable, including rigs worth several hundred pounds I found this annoying and unnecessary for a simple antenna, especially as the ethos is for amateurs to experiment and modify things. It certainly wasn’t glued for weatherproofing as it would be have sealed all round, it could only have been to stop it being opened. I could understand this if it were protecting some patented, copyright design but it wasn’t, behold the magic within the “special matching unit” nothing but a simple un-un which I suspected already. 

Anyway I was able to tighten up the simple screw and bolt and ironically I used some epoxy resin on the screw head to try to stop it coming loose again. Hopefully this loose terminal was the reason for the poor performance last time.

I am looking forward to having another go from operating in Scotland this year with some more experience I hope to improve on last years efforts

While tinkering in the shack I kept the FT857 on and tuned around looking to give points away for the VHF Field Day and made a number of decent DX contacts using just 30W

Some PMR fun

It was the Dambusters Hamfest on Sunday. Sadly the weather was awful with heavy rain all morning so was only there for an hour. Despite the flying visit I managed to pick up a couple of old PMR radios for conversion.

One is a Tait T535 2-Channel Mobile PMR which can be converted to amateur 2m operation relatively easily. Details are available on the internet including David Pye’s website. I intend to use this unit to set up a permanent APRS internet gateway/digipeater.

While walking around with the Tait I attracted the attention of another seller who had some PYE MX294 units for sale. One was already converted to work on the 25kHz spaced simplex and repeater channels with CTCSS. The other was only partly converted and they came with a box full of spare boards and mounting plates and an original microphone and speaker. I wasn’t really looking for any more units but seemed a bargain for £20.

The PYE MX294 was indeed converted and seemed to work fine on the simplex channels, and while it was receiving and transmitting on the repeater channels it wasn’t transmitting the CTCSS sub-tone. With some sage information from Ian MW0IAN on twitter it was a simple issue to solve, using the oscilloscope I confirmed the tone was being generated it was the pot controlling the level that was set too low.

I have made a couple of QSOs with it and had some good audio reports. It is capable of 25W but has been set to 10w output so may increase this in time. Stewart M0SDM from the club was kind enough to video a QSO to illustrate the audio. Impressed by the quality of the unit I have ordered a second hand copy of the PMR Conversion Handbook as published by the RSGB from eBay so I can finish the the other MX294 as information on the net is a bit sketchy.

The Tait is proving a little more involved. It has a diode matrix board to set the Tx/Rx frequencies which has been set incorrectly. It also had a CTCSS board that was hard-wired in rather than socketed which I have now removed. The information of which diodes to link/unlink is freely available, however I decided to create a small windows utility to simplify the calculation. This is available for download from my website, it is supplied as is and with no warranty.

There are a number of modifications to add multi-channel to this radio by the use of eproms, but I am considering devising an Arduino solution to allow setting of frequencies and possibly selection of CTCSS.

FTDI still bricking chips?

I doubt anyone missed the recent FTDI driver controversy.

The Scottish company Future Technology Devices International (FTDI) released an updated version of their USB-to-Serial driver for Windows on their website late September and last month the driver became available via Microsoft Windows Update. It soon became apparent that these new drivers could ‘soft brick’ counterfeit and software-compatible clones of their chips by re-writing the USB product ID (PID) to “0000”. This action prevents the chip from being recognised by drivers of any operating system, effectively making them inoperable unless the PID is changed back. This clumsy and ill thought out measure was intended to protect its intellectual property.

The ability to reprogram the USB Vendor/Product IDs is a feature of FTDI devices offered to equipment manufacturers and so most bricked devices could be reset by using the downloadable FTDI utility.

Obviously there was much outrage from the hobbyist community and FTDI were roundly criticised and as a result the malicious driver was supposedly removed from the Windows update system.

Or so I thought…

I am currently constructing another data mode interface for some experiments with FreeDV. It requires the usual PTT control driven using the RTS line from a serial port. I purchased a couple of simple TTL level interface boards on eBay which claimed to use FTDI chips.

I built up a little scrappy veroboard circuit with an open-collector drive transistor and plugged into the shack PC and everything seemed okay as this PC already had an older FTDI driver installed.

I am using another computer for the FreeDV experiments and plugged the board into this thinking it too already had a safe FTDI driver installed but instead it brought up the installing driver dialogue and appeared to go online and download drivers and install them. I really didn’t pay much notice as I wasn’t too worried as any malicious drivers had supposedly been pulled and sure enough after installation everything appeared to work, the port appeared in device manager.

I unplugged the board to make a slight change and was surprised when I plugged it back in the PC the driver installation dialogue reappeared followed by an error message saying driver couldn’t be installed and contact the manufacturer.

The serial port now appeared in the device manager with a yellow exclamation mark saying no driver installed error 28. Examining the device details showed that the VID was still 0403 but the PID was 0000 it had been bricked! Unfortunately not soft bricked as I have been unable to reset the chips PID using the FTDI utility.

I am not exactly sure what has happened but still a case of beware when it comes to FTDI devices and I shall be avoiding them from now on.

Datamode Interface built

This weekend I finally got around to sorting out my digital/data mode interface for the Yaesu FT-857D.

To transmit and receive digital/data modes you need to connect the radio audio in/out to the computers sound card in/out, the computer then runs the necessary software to encode/decode the signals. I want to try out WSPR, PSK, JT65 and some SSTV for starters I have spent too long just receiving and decoding…

There are a number of inexpensive commercial interfaces available, but many of them use the same basic design originally intended for eQSO/Echolink operation. I nearly succumbed but I had built an eQSO interface many years ago when using PMR446 and had most of the parts to build another.

I nearly took the easy route and got a commercial one since connecting up some home built circuitry to a £20 hand held is slightly less daunting than plugging it into an expensive rig!  My original interface has been modified and reused over the years and was a bit of a mess, but being brave I decided I could tidy it up and I couldn’t really damage anything if I took my time… actually the truth was I discovered I didn’t have the necessary optocoupler IC so couldn’t build a new one just yet…

A simple internet search for digital/data mode interfaces will bring up a great deal of information, schematics and ideas for home brew solutions. The basics can be found here for example.

The simplest form of interface is just a simple direct lead with the transmitter operating in VOX mode. However levels can be a problem as the line/speaker output from a computer can be too high for a transmitters microphone input. Also connecting a radio to a computer directly can lead to problems with ground loops and interference.

The computer can be made to control the Push-To-Talk (PTT) on the transmitter using a serial port with the software controlling one of the handshake lines (RTS/DTR) Some data mode software support CAT to allow control of the PTT as well as tuning the transmitter, but the serial port method is more universal.

The preferred interface, and the one I had built isolates the computer from the radio by using two audio transformers and an optocoupler. There is no direct connection between the two devices so keeps interference to a minimum.

I could have used the microphone and speaker output on the radio, but the FT-857D has a convenient data connector on the back. This is a 6-pin DIN socket as used by older (PS/2) computer keyboards/mice. Note the diagram shown in the FT-857D manual (as below) is the view as you look at the socket.

FT-857D Data connector as in the manual

The connector has two “data” outputs but they are simply fixed level audio outputs from the receiver. The one of interest for most modes is the 1200 baud output (the 9600 baud output is more akin to a discriminator tap and is only of use for FM packet) There is a data-in (TX-audio) and a PTT control line.

Like many people I initially thought I could cut a lead off a mouse/keyboard and repurpose it, however I discovered most only use four wires and they don’t use the necessary pins! You might be lucky especially with older keyboards or alternatively if you have an old keyboard extender cable they usually have all six wires present. Alternatively the plugs are readily available from the likes of CPC/Farnell.

I had a hunt around in a junk box and located a suitable keyboard extender cable. I chopped off the useless end and metered out the pins to identify the appropriate wires. Remember when looking at the plug the pins are swapped left-to-right compared to the diagram which is the socket view.

Well here is the insides of the interface.. and as you can see I completely failed to tidy it up! Not my best work, but I did put it in a new box and I did tape up all those unused wires!

The messy internals of the interface

One annoying issue I had was the audio from the computer wasn’t getting to the radio, it worked and scoped out okay when out the box, but in the box it stopped working. I suspected a bad solder joint and redid them all, but still showing the same intermittent issue. I did notice flexing the board slightly out the box cause the fault and soon located a track on the vero-board which appeared to have a hairline crack, I couldn’t see anything obvious but a wire soldered along it cured the issue.

It looks better with the lid on..

I soon had it connected up and fired up WSPR

Computer, radio and interface

It was straight forward setting up WSPR to use a combination of CAT for tuning and the RTS PTT control and soon had some encouraging results, in fact these are some of the spots of my 5W signal on 10m/20m and 30m, I was grinning from ear to ear!

10M Spots

20M Spots

30M Spots

I had simply set up the FT-857D for basic USB transmission, however it does have a dedicated DIG mode, the manual refers to setting it up for RTTY/PSK, I very briefly had a go with PSK31 and Digipan and was successfully decoding signals and put out a few CQ calls (again on 5w) but got no immediate response. However checking PSK Reporter later it seems I had been heard by VC3S, OH1FOG and ES1JA maximum distance was around 3260 miles

Looking forward to spending some more time experimenting with the data modes.

Welcome new readers

Firstly welcome to all my new readers via AmateurRadio.com

I was very surprised, shocked even, to be asked to syndicate my postings to the site. Hopefully my posts will be of interest to a wider audience.

I have been a licensed amateur operator for six months currently on the bottom of the UK licence structure, the so called Foundation licence, but am hoping to take the Intermediate licence soon.

The main driving force for wanting to progress is I want to build things, to experiment and learn.

In a natural extension/diversion from my day job I have already been experimenting with the Arduino platform, initially with the intention of developing a High Altitude Balloon tracker but more recently with other radio related projects including a satellite tracker and using it in conjunction with DDS modules for WSPR/QRSS purposes. I have just built a dedicated Ultimate 3 QRSS kit from Hans Summers (G0UPL)

The DDS module are particularly interesting and I have some tentative plans for an Antenna Analyser and a Power/SWR meter capable of working down in the mW range which I stumbled across on the website of Loftur E. Jónasson – TF3LJ / VE2LJX. This is of particular interested to low power QRP operating. Did I mention that I have joined the GQRP Club?

Progressing from a couple of Baofeng VHF/UHF handhelds (which I hardly use) to a proper rig last month with the purchase of a Yaesu FT-857D I have been dipping my toe into the frightening world of operating!

It is common for new amateurs to be “Mike Shy” and I admit to suffering terribly. Not being the most outgoing or confident person being confronted by a barrage of rapid fire abbreviations, codes and etiquette it took a while before I had the courage to key up.

I plucked up the courage to have an attempt at some of the RSGB UKAC VHF evening contests and after gaining a bit of confidence I ventured properly onto the HF bands this weekend making a few simple signal report QSOs.

I should like to thank all those who have been patient with me as I fumble along.

Because of the shyness the use of data ‘digital’ modes is a strong draw since it uses computers and you don’t have to talk! I am salvaged some suitable connectors to build a new computer data interface for the FT-857D, I built one several years ago but it got slightly cannibalised when experimenting with an ARPS gateway.

I can afford a commercial interface but why should I pay over the odds for something I can easily build myself? The desire to homebrew isn’t just driven by cost, but lets be frank this can be an expensive hobby! Nothing gives more satisfaction when something you built works.. and yes they may be famous last words.

Ultimate3 QRSS Beacon kit built!

At the start of the year I did some experimentation with cheap DDS modules based on the Analog Devices AD9850 connected to the Arduino board, making a rudimentary WSPR transmitter prototype.

My current licence restrictions prevent me using anything home-brew for transmitting except for commercial kits. So I ordered an Ultimate3 QRSS beacon kit from Hans Summers (G0UPL) thinking that it would be okay. I subsequently learned that any commercially available kit must satisfy IR 2028 which is all a bit vague and unclear but sadly I don’t believe this particular kit does.

All was not lost, building this kit should more than satisfy one of the practical assessments of the intermediate examination, which will get me around this problem.

The Ultimate3 kit is extremely popular and so I had to wait a little for delivery and it arrived on Friday. After the last few weekends of non-radio activities I had planned to get my antennas backup and do some proper operating. Like many people I had been forced to take everything down due to the barrage of storms and high winds the UK has been experiencing recently.

A tidy workbench

Saturday saw no let up in the wind, so I decided to spend a few hours building the kit instead.

The instructions were extremely clear and straight forward and soon had it built up, though it is high time I invested in new soldering station. I have a basic Antex 25W iron. I cannot remember exactly when I brought it but it is well over 10 years ago.  It was more than adequate to build this kit and for soldering connectors but I could do with something adjustable and more comfortable.

Taking shape

The only issue I had was winding the first toroid, 25 turns later I realised I had wound it the wrong way round so the leads didn’t line up with the holes in the PCB. I could have made it fit but nope I will do it properly so I unwound it and did it again.

I also made the mistake of not scraping the enamel off the toroid wire and tried the heat it and bubble it off method, except I think my iron just isn’t hot enough so ended up using a piece of wire wool to remove the enamel.

Lessons learned I soon had the other three toroids correctly wound and wire prepared for the low-pass filter board.

Close up of the LPF

   A final visual check and powered it up and it worked first time!

All built
It works!

Setting it up

Full of confidence I grabbed my trusty GPS module which has been pressed into service on a few Arduino projects including the HAB project. Quickly soldered some connecting wires and powered everything up.. all was well or so it seemed when the display suddenly went blank, backlight was on, just no characters.

Pressing the button I occasionally got some random characters and a flashing cursor! I de-soldered the GPS and still nothing. I suspected the display was faulty but trying it on the HAB prototype board confirmed it was okay. I checked the display connector continuity and everything appeared okay.

Out with the oscilloscope I started probing, everything checked out. Crystal was oscillating and data pulses on the display control lines. Then I checked the supply pin on the display and it was only reading 4.1V, this under-voltage would explain the odd display behaviour.

PSU output was 5V, micro-controller was 5V, DDS module had 5V. All very puzzling till I removed the DDS module and spotted a discoloured track on the PCB, touching it with a screwdriver and the lacquer fell away revealing a tell tale scorch mark, somehow I had made a nice resistor!

Burnt track to right of micro-controller

A quick wire link soldered in place and everything was back to normal.

What caused it? Checking the de-soldered GPS connecting wire I spotted a stray single strand of wire on the ground wire. I suspect this must have shorted to the adjacent 5V line and since I was using a nice beefy ex-PC PSU as a bench supply it had popped the track without the hint of a flicker. The GPS has been rewired properly and is working nicely, now to connect a dummy load and experiment some more.

Sunday was a lovely day, wind dropped so antennas have been put back up and I took the opportunity to tidy up the installation a bit. I also dug out an old fibreglass pole to put the M0CVO HW-20HP back up. I didn’t get to do any operating in the end as by the time I had done this and made up a couple of decent patch leads it was time for roast beef and all the trimmings and an evening in front of the TV.

The HW-20HP back up

Putting PL259 and N-Type connectors on coax is also part of the intermediate assessment, so perhaps I should have videoed making up the patch-leads as proof 😉

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.


Breaker – Breaker

It is my long held intention to become a licensed radio amateur, but with limited spare time over the last few years I simply haven’t got around to it. I have the necessary technical knowledge but don’t have the practical experience. I have dabbled with PMR-446 transceivers but they don’t allow many ‘legal’ experimental opportunities.

So when my family asked me what I wanted for Christmas I said “You can buy me a CB Radio” and they did!

It is a Midland 78 Plus Multi it has the legal 40UK/40EU channels, plus it can be set to European and North American frequency bands.

It isn’t the first CB Radio I’ve used, back in the early 80s when I was at school I managed to blag the use of the a friends set up with a small magmount antenna on a biscuit tin joined in the new craze! My handle was Morroco Mole 😉

I am experimenting with the set up at the moment, powering it from a modified PC supply which is giving out a nice stable output a little over 12V. I have constructed an inverted-vee antenna out of some spare cable. It is currently in the middle of the back lawn, the central mast is only around four meters high and the elements are set approximately 45 degrees to the vertical. It has an ‘ugly’ air choke and using a £10 SWR meter I’ve tuned it and the worst ratio I get is a little over 1:1.5.

It all seems to work but at the moment I don’t seem to have anyone to talk to on either the UK or the EU/CEPT channels. I’ve heard the odd transmission on some channels but they have been from nearby Mansfield and Nottingham.

I have monitored CB frequencies before with my scanner with limited success and was hoping for better with a proper CB radio. It is still early days but I think I may need to rethink my antenna until I get around to getting my mast sorted. I might have to stick the inverted-vee in the loft.

I was tinkering on Sunday afternoon and I switched the radio over to AM just as some more “Superbowl” skip came in, they really are amazing to listen to! 

RTLSDR – Part 4 – Cutting out the noise using a simple coax filter

One major limitation of both the FUNCube Dongle and the RTL-SDR based receivers is the poor front end filtering.

Connecting a wide band antenna such as a discone to the input often results in a spectrum full of noise which swamps the target signal.

In most cases the noise is actually a result of out of band high-power transmissions such as FM and TV broadcast stations and pagers which overload the receiver and get mixed in to the tuned signal. The effects can be mitigated to a degree by turning down the gain of the receiver but at the expense of sensitivity to the signal you are wanting to receive.

The FUNCube Dongle in my experience seems especially susceptible to the effects of FM broadcast signals when trying to monitor the 2-Meter amateur band. I’m not sure if the current weather conditions were the cause but yesterday evening it seemed the breakthrough was especially bad. So I decided to experiment with a simple coax ‘notch’ filter.

There are two approaches to filtering, one is to use a band-pass filter which passes frequencies within a certain range and rejects (attenuates) frequencies outside that range. The other is to use a band-stop filter that passes most frequencies unaltered, but attenuates those in a specific range to very low levels. A notch filter is a band-stop filter with a narrow stop band.

As it happens you can make an effective filter using ordinary coaxial cable which will generate a sharp, deep notch which can be used to attenuate the unwanted signal.

A coax stub notch filter can be made if you connect the antenna to the receiver using a T-connector then attach a piece of coax of the appropriate length (the stub) to the remaining connector.

I happen to have a fair bit of 10Base2 Thin-Ethernet cabling (RG-58U) and connectors which were repatriated from a previous employer over a decade ago. The network cards they used came supplied with lovely long high quality 10m patch leads with moulded BNC connectors and a T-Connector and terminator! Even back then the 10Base2 was completely redundant as the network infrastructure used Cat-5 cabling.

So I dug out one of the T-Connectors and a BNC connector and set to work, connecting the antenna and receiver was simple, all I needed was to construct the coax stub.

To calculate the length of the stub you must first calculate the wave length of the desired ‘notch frequency’ for the FM band a good central notch frequency is around 100MHz.

The wave length is found by dividing the speed of light (which is around 300,000km/sec in free space) by frequency

So for a 100MHz signal the wave length is 300/100 = 3 meters.

However the speed of the radio wave in a coax is affected by the material it is made of, this is called the velocity factor. For most solid polyethylene coax (e.g RG-58) this is 66% (a handy table can be found here)

So multiplying 3 x 0.66 = 1.98 meters

The coax stub is 1/4 of this wavelength = 1.98 x 0.25 = 0.495 (49.5 cm)

So cutting a piece of coax of that length, connecting it to the T-Connector and leaving the other end open completes the filter. The filter will have maximum attenuation at 100MHz (assuming the coax is cut accurately) and the attenuation will fall off away from this ‘notch frequency’ (Note the filter will also notch at 300MHz, 500MHz and so on)

Interestingly it seems my RTL based SDR receiver is more affected by pager breakthrough, so I plan to make an alternative stub, adjusting the calculation to use 153MHz as the notch frequency to reduce this. It is possible to add more than one filter at a time using multiple T-Connectors and alternative higher performance designs for coax notch and band-pass filters can be found with a quick Google search.

I made a quick video showing the effectiveness of this simple filter which took about 10 minutes to make!