Comet’s CHA-250B is an interesting antenna. It claims (and delivers) continuous broadband coverage from 3.5MHz to 57MHz with no ground radials. Comet’s website admits this is not without its compromises of course:
“If you have the space, budget and desire to erect a full size antenna system we suggest you do so… bigger is better…however…if you live in an antenna restricted area and must manage with antenna or space restrictions or you simply wish to operate incognito you will be forced to make significant antenna compromises. The CHA-250B will make the most of these circumstances!” – www.cometantenna.com
The antenna is shipped in sections with all mounting hardware required, and bolts onto a 1” to 2” diameter mast. It is an easy, 10 minute job to assemble the antenna with basic tools.
We tested this antenna mounted on a 5 metre steel pole to elevate it above the ground. A sweep of the antenna with an antenna analyser confirmed 1:1 SWR across its entire operating range. The design has been well documented (for example by Martin G8JNJ http://g8jnj.webs.com/cometcha250b.htm) with a lossy transformer dissipating heat. Although the numbers are all there we were interested in getting some first hand experience. We compared CHA-250B signal strengths on 40m with a dipole (apex approx 7 metres AGL) and on 20m with a TH6 3 element beam at 10 metres AGL (not a fair comparison!!). We used an FT1000MP MkV (200W PEP). Propagation was typical for this time of year.
Results – 80m
First up we had a surprise – tuning into an 80m net we called in on the Comet and received good signal reports (59) from stations in southern NSW and in VIC. Good range (1000km)! Our 80m horizontal is tuned for the DX portion of the band and a pretty ugly match on the net frequency, so we didn’t risk stressing the FT1000′s tuner. Comet wins this one thanks to its broadband nature.
Results – 40m
40m gifted us with an opening to Spain, with a couple of stations calling CQ through monoband beams with lots of power. We had success replying on the 40m dipole with a few repeats but the Comet could not be heard when we switched to it. 40M dipole wins by an unknown margin.
The biggest difference between antennas was found talking to a VK2UW approx 400km north of us. Chris had a double bazooka antenna pointing our way and our dipole was facing him. 59 report on the dipole that dropped to a 33 on the Comet. Being a vertical, the Comet is not suited to a short distance skywave contact due to its low angle of radiation.
We also spoke to two club members via ground wave (approx 8km away), there was not much difference between the two antennas, maybe 1 S point.
Results – 20m
No surprises here, TH6 wins. G8JNJ’s page says the CHA-250B is -3dB relative to a ¼ wave vertical. The TH6 claims 9dBi gain (this is free space gain, simulations suggest up to 12dBi depending on the takeoff angle). One would therefore expect the TH6 to return signal strengths on average 15dB (3 S points) better while having a lower noise floor due to its directionality.
We tuned into the Southern Cross net and called in, then switched between the antennas. Signal reports in both directions were typically S9+20dB with the TH6, and S7 to S9 with the Comet. The encouraging thing here was that basically all the participants could hear us on both antennas! TH6 wins the signal report obviously but the Comet is a viable antenna. In general tuning around we could sometimes hear stations on the Comet that were not audible on the TH6 due to direction. The Comet could therefore be used as a searching antenna and when a station is identified the TH6 swung around for a more effective contact. Since the Comet is broadband it will easily cover the TH6′s 3 bands (20,15,10m).
The Comet is a relatively quiet antenna, a description often stated as a benefit but what that really means is lacking in signal strength. However, the useful measure on receive is signal to noise ratio and this was on par with other antennas. Listening to stations on the Comet was no more or less legible than our other antennas. It was rare that we couldn’t hear a station on the Comet compared with our other antennas. The main disadvantage is the transformer loss on transmit.
On the plus side, the Comet CHA-250B is an omnidirectional antenna. It operates on every HF band besides 160m. No tuner is required, band changes are instant. It was very novel to be able to say “let’s try 30m” and put out a few calls without changing antennas. 80M performance surprised us with clear and easy contacts for such a short antenna. 20M showed that DX was straightforward when conditions were reasonable.
If you can’t put up a full size antenna, this is an attractive product. The broadband nature is in some respects a treat, it allows for band hopping and would lend itself to experiments in ALE (Automatic Link Establishment) if desired.
Antenna graciously provided for testing purposes by Benelec Pty Ltd
Geoff VK2TGO recently gave a lecture on an end fed antenna for 40-20-10m. This antenna is fed using a 9:1 unun, just under 13 metres of wire and a loading coil near one end. The antenna wire itself can be deployed in many ways: vertically, as an inverted V (with the feedpoint at one of the low ends) or as a sloper.
This is a light, versatile antenna for portable use!!
Lecture slides and construction details:
ILLW – International Lighthouse and Lightship Weekend
Personally, I think ILLW stands for It’s Likely Lousy Weather! This year was no exception. Dom VK2JNA, Geoff VK2TGO and Geoff VK2AVR assembled at Staging point #1 (the coffee shop) just before 8am Saturday morning. We continued to Staging point #2 (Palm Beach carpark) where we met the NPWS volunteers who would open the lighthouse for us. After 91 metres of vertical ascent carrying our gear, the weather greeted us with the first shower as we crested the headland. Undeterred, we hung an OCF dipole from Barrenjoey’s man-made monolith and were on the air in time for 10am kickoff. 40m was the band of choice, and several club members called in on our home frequency of 7045kHz. We then alternated between CQing on one frequency and “search and pounce” around the band. A voice keyer for the FT-897D made CQing an activity of leisure, just wait for a callback and then grab the mic!! To be fair to the weather, the afternoon improved and no more rain was encountered until the evening. Dom VK2JNA gave spontaneous tours of the lighthouse keeper’s cottage to approximately 100 interested members of the public, including our radio station and explained what ILLW was all about.
Dom VK2JNA and Geoff VK2AVR stayed overnight, and experienced a wild rainstorm around 2am. The keeper’s cottage, being built of hefty sandstone blocks, simply shrugged it off!
The weather on Sunday morning continued to be unpleasant, and NPWS decided not to provide the usual Sunday tours of the lighthouse. To our surprise, club member Derek arrived mid-morning looking very wet! We warmed him up with a cup of tea but he had to go home for a change of clothes shortly afterwards. Dom VK2JNA gave some more tours to the public, Geoff VK2TGO came back up and we also saw Jeff VK2MZZ, his son Tom VK2FTOK, Patrick VK2PN and Steve VK3TDX. The weather steadily improved throughout the day meaning we could pack up and descend without worrying about rain gear.
Overall, we had 160 contacts including 26 lighthouses!! We had contacts into Europe (Sweden, Netherlands) and North America (Alaska, east coast USA).
Sadly, it’s a long time until the next one!! We will get busy writing out QSL cards in the meantime. Another great event and many thanks to NPWS for allowing us access to Barrenjoey, Geoff VK2TGO for organising with NPWS, Dom VK2JNA for manning the station for the whole weekend (!!) and doing some great PR work for the lighthouse and our club, and thanks also to the visitors who came up.
Total contacts: 160
RD contest contacts: 67
DX contacts: Sweden, Netherlands, Alaska, East coast USA, Indonesia
Mal’s capabilities as a hacker are really on show here with his balancing scooter getting some airtime on YouTube!
For those who frequent our YouTube channel you may remember Nick Hakko giving the club a lecture recently. Much to our excitement Nick’s work has recently gotten him into the press.. Check out the article here Sydney watchmaker is refusing to call time on his career
If you missed his video check it out here
Well my first year as president of the MWRS has flown by, and I am still humbled by your approval and trust in me taking on the role.
I have always loved technology, tinkering with gadgetry and socialising and this club represents and promotes all 3 of these things to the wider community. Sure we are primarily an Amateur Radio club, but as our member base welcomes younger members we are seeing a brilliant knowledge exchange of electronic wizardry being handed down to the next generation while at the same time we are seeing the wonders of computer being demonstrated the other way. My goal is to continue to promote this and our membership continues to welcome in experimenters of all ages into the future.
It’s been a full year of events, with the usual lighthouse weekend, bbqs, training weekends, camping trips, JOTA and working bees continuing to be a huge success. I cant thank you enough for making these possible.
I was also hugely excited (as you will no doubt have seen during our meetings) to see new events like the pirate / flagpole weekend (arrr) and the Maker Faire pop up in the calendar. Good fun stuff!
This AGM we will see a few of our longest serving volunteers retire their roles, and I think we all owe them a huge debt of gratitude. Some of them have been diligently handling their areas of responsibility for longer than I have graced the planet! I would ask you all take some time to reflect on their long work and thank them for helping us keep the show running. I would also like to take the time to encourage new players to step up into those roles, we may joke around that non-attendance will get you a role but in all seriousness contributing to the club through a formal role is a highly rewarding thing to do and and the perks are great (Apparently we have a private presidential jet, but its been out of order this year. Weird).
I have had a blast this year, and certainly hope you all agree that this club just keeps on getting better.
The last year has been pro-ductive for our repeater site, with several notable up- grades. The biggest up- grade was the replacement of our main VHF antenna. We were using a 5/8 wave- length amateur-grade verti- cal that had been in place several years, but the per- formance was degrading. Inspection revealed it was full of water! We have re- placed it with a dual folded dipole array. This has re- stored the performance of the antenna system and, being commercial grade, should last for many years to come. Shortly after this up- grade, we had reception reports as far away as Jervis Bay (Dave VK2AWJ). Par- ticular thanks to Tim VK2BT and Yves VK2AUJ for their assistance.
APRS also received a big upgrade, our iGate now uses a single folded dipole at the top of the tower. The previ- ous iGate antenna was a magmount on the repeater
hut roof! With the relocation of our antenna to the top of the tower we are now cap- turing more packets directly and compensating for the loss of VK2US-1 in the CBD. A 3-fold improvement was immediately apparent when we moved the antenna, and some fine-tuning of the audio levels improved reception even further.
Our analogue VHF repeater now has a MicE interface. By interfacing an APRS transmit- ter with your normal mobile radio, you can send MicE packets on the tail end of your over. The MicE inter- face will receive these (and mute the retransmitted audio to save our ears). This is a great alternative to having a second transmitter, second antenna etc on the car. Credit to Mal VK2TMF for the MicE project.
There has been renewed interest in getting our 6m repeater back on the air. Work has been going on
behind the scenes in resur- recting our 6m repeater (Steve VK2KFJ). We are also designing a new an- tenna for the tower, most likely a 1⁄2 wave flowerpot.
On the DSTAR side, Matt VK2RQ has recently installed IRCDDB to improve callsign routing, and a new linking module allowing us to con- nect to DCS and XRF reflec- tors in addition to the Dplus support we had already.
Finally, we are still chasing an interference problem that happens when our VHF ana- logue and DSTAR are used concurrently. It does not appear to be an issue either for this kind of work. Thanks again to all who donated so that our club could own this great measurement tool.
The MWRS Education team continued to provide AOCP training and Assessment services to the Amateur Radio community. A number of Foundation revision and assessment Saturdays were conducted at Terrey Hills club house with a number of others completing their higher level assessments. Please spread the word that MWRS has this capability. If there is a friend or a relative that has shown an interest in Amateur Radio, let them know about the ultra-convenient one day format of the MWRS Foundation revision and assessment day. The MWRS Education team also offers a training course for Regs followed by the regulations assessment. This course and assessment takes approximately 4 hours. Of course we conduct upgrade assessments for those wishing to do one or more Amateur radio assessments. The Education team conducted a total of 66 assessments over a number of different venues during the last twelve months.
A special thank you goes out to our clubhouse custodian for allowing us to use the Terrey Hills clubhouse for the Education and assessment days.
My first JOTA
I attended my first JOTA event at MWRS club last year. I have got to say, I found it a hoot, assisting in getting the scouts and guides around all the different activities. I would recommend it to all and suggest even if you are around for a couple of hours between your other weekend commitments consider assisting I think you will enjoy it. I’ll be there again this year and my partner Katrina VK2FKAP with her pod coffee maker, serving coffee to the help. If you have an hour or more free, come on over and join in the JOTA activities.
Social Report for Up the Stick!
We love a good BBQ at the MWRS and thanks to my blowing the budget last year with the Coles Country Beef sausages we have had a few BYO BBQs to get us financially back on track. BYO BBQs are extra special as the personalities of the members really shine. There was a circular sausage that looked like a sleeping snake. I tended to go for a steak and mustard. With all sorts of variations in between. The BBQ itself continues to serve us well however at some time in the future it will go where all great BBQs go, into the sky and BBQ heaven, and a rebuild will be required for this iconic piece of kit. (I have some cad plans of a super duper BBQ if we decide to proceed this way) I would also like to make mention of the XLYs and Harmonics that also attend and make these events extra special.
Flag Pole Competition – Yes I know it overlaps with contests reports but it is social contest.
Firstly a correction from last year’s edition of ‘Up the Stick‘. The 25 foot Chinese made flag poles we purchased are in fact only 24 foot long. Thanks to Tim VK2BT for actually measuring the unit. They must have different rulers in China. In any case the event was a success. I managed to scare my wife Annette, VK2FOXE, when the Jolly Roger Flag that arrived at her work from FleaBay, as she though we were being targeted/stalked in some way by some one or group. No need to worry dear, just your husbands antics.
Despite the scoring regime I think Geoff, then VK2MIA, now VK2AVR took out the fun side of the comp with both a flag on his pole, pirate hat and a protest from onlooker saying that he couldn’t do what ever it was he was doing. Well done Geoff!
As with previous years another well attended event at the Terrey Hills Tavern in a similar location to last year which worked extremely well. It’s a good arrangement as you purchase what you want when you want and simply eat it. Stories are swapped and laughter was a plenty as it is every year.
Jim (Inset callsign) left us to go back o the UK and unlike last year where we made a ‘Bye Jim’ sign out of sausages on the BBQ, photographed it and sent it to him Dom suggested that it may be prudent to hold the event before Jim actually left. Another great day and BBQ. It is the MWRS way, thanks Dom.
Firstly, I have enjoyed my two years a social officer, however the promised groupie like following of YLs has not been as expected.
I was hoping in the following year we could sneak in at least two more BBQ and in conjunction with the Publicity officer see if we can not make this flag pole contest a growing event. On top of that I was also thinking of some last minute dinners around Terrey Hills before the club meetings could also be encouraged.
Everyone has a different idea of Lazy Sundays.
For Tim VK2BT, Geoff VK2AVR, Charlie VK2GMT and El Presidente (Carlo) VK2MXC, this past lazy Sunday was an opportunity to chase some interference problems. Our VHF repeaters (analog and DSTAR) have had an issue recently – when both are keyed there is audible “splatter” on the analog tail (it sounds like somebody blowing a raspberry, how rude!). A decent incoming signal will swamp this and give clear audio but the repeater tail will have raspberry on it. On DSTAR this manifests itself with a garbled “R2D2″ audio if your signal is not particularly strong and one presumes on analog that if you’re a weak signal you will struggle as well.
Using a spectrum analyser we characterised the problem. Because of the broad nature of the noise it seemed like an intermodulation problem. This happens when the radio signals mix in something non-linear – anything from another radio transmitter to a rusty bolt on a tower (the metallic junction can act as a diode and produce non-linear rectification products). By isolating our equipment in the hut one by one we searched for the cause. Nothing we disconnected made any difference. Our repeaters are also well filtered, so we didn’t suspect a problem there. We proved that to be the case by connecting their combined output to a dummy load instead of the antenna. Blissful silence and no raspberries. Not us. The problem remained even when we used our secondary VHF antenna on the tower, both primary and secondary are in good condition and swapping made no difference so it’s unlikely to be an antenna issue. Our attention then turned to the tower. The signals must be radiating, mixing with something else to cause the noise and then the noise is being received on our antenna again. Climbing it at such short notice was out of the question, but we had an idea. What if we pointed a directional antenna towards it and sniffed around, could we see the distortion still?
After a quick trip to the club rooms, we arrived back on site with what we thought was overkill – a 9 element loop fed Yagi constructed by Chris VK2YY. This was affixed to a temporary mast and connected to the spectrum analyser. After receiving some funny looks from passersby we tried to detect the interference. As it turns out, we could just barely make out a difference on the analyser screen when the interference was present and the Yagi pointed directly at the top of the tower. When the Yagi was swung away from the tower we couldn’t see any spurious signal. Given the signal was so low, we could barely see it on the spectrum analyser screen this time, the radiated power of the interference is very small but enough to upset the sensitive receivers in our repeaters.
The cause? It could be a rusty bolt, it could be some other radio equipment mixing and re-radiating distortion products. It is a shared site after all. Our next step is to contact some of the other site users and see if we can find out more about their radios.
As some of our members will be aware, I have been building an Elecraft K2 kit radio, and I completed construction this Easter weekend.
When I ordered the kit, I didn’t order the internal ATU or the 100W amplifier option, as I already have an Elecraft KXPA100 amplifier with KXAT100 ATU. However, this amplifier is designed to work with the KX3 radio, and so to get it to work with the K2, I would need to build up a keying circuit.
Fortunately, Elecraft published on their website a neat solution for this. It consist of a couple of transistors mounted on some perfboard, which slots into a PCB header socket intended for the K2′s transporter interface module. Because I don’t have this option, my radio didn’t have the required header socket, but luckily the local electronics store sells suitable headers as part of their Arduino range. I picked up one of these headers, together with the other requisite parts.
The article on the Elecraft website provides a schematic, and some photos of the finished board, but they didn’t provide detailed information about how to build the circuit onto the perfboard. Fortunately, the circuit is quite simple, and wasn’t too hard to work out, but I decided to draw up a diagram which provides a better view of how the components are laid out and the physical connections are made.
The dotted lines show connections running under the board (except the gate lead of the 2N7000 JFET — that runs under the transistor, but along the top of the board).
I built the circuit in the following sequence:
1. put the 8 pin male header under the board on the left side, and solder one leg of the 2.2k resistor to the pin sticking out the top of the board at position A8. This then holds the header in place. The other leg on the 2.2k resistor goes through hole B5.
2. Sit the 10k resistor sitting up on end in hole C5, bend over the top leg and stick it through hole D4. Bend the legs underneath the board to hold the resistor in place.
3. Put a piece of hookup wire (eg. a component lead off-cut from when you built your K2) through hole D3. On the top of the board, bend the wire so it reaches the header pin 2 sticking through hole A2 and solder it to that pin.
4. Mount the right-angle molex receptor on top of the board through holes E1 and F1. Under the board, bend the hookup wire you installed in the previous step so it reaches the receptor pin sticking through hole F1, and solder to the receptor pin and trim. This will hold the receptor in place.
3. The Gate and Source legs of the 2N7000 JFET are the left two legs, if you hold the transistor with the flat face towards you. Bend those two legs backwards, and put the remaining leg (the Drain) in hole C1. Bend the Drain leg under the board down towards the resistors to hold the transistor in place.
4. Solder the Source leg (the left one you bent back) to the header pin 2, which sticks out of hole A2, and trim. Careful not to dislodge the hookup wire that is also soldered to that pin. Trim both the transistor lead and the hookup wire.
5. Solder the Gate leg (the middle one you bent back) to the header pin 1, which sticks out of hole A1, and trim.
6. The drain of the JFET which goes down through hole C1, and the resistor legs sticking through holes B5 and C5 should be bent so they all meet up. Solder the three leads together and trim.
7. The Emitter leg of the 2N2222 BJT is the left-most leg, if you hold the transistor with the flat face towards you. Bend that leg backwards, and put the middle leg (Base) through hole C4 and the right most leg (Collector) through hole C3. Solder the Emitter leg that was bent back to the header pin 4 sticking up through hole A4.
8. Bend the Collector leg sticking through hole C3 so it reaches the molex receptor pin sticking through hole E1 under the board, and solder the leg to the receptor pin.
9. Bend the Base leg sticking through hole C4 so under the board it meets the 10k resistor leg sticking through hole D4, solder the two legs together and trim.
10. Make up a short cable (3″ or 4″) with the molex plug on one end. Solder the side of the cable that connects to the pin closest to the side of the board, going through hole F1, to the ground lug on the RCA connector.
11. Install the RCA connector in the hole on the bottom chassis labelled “XVTR IN”. Solder the other side of the molex plug cable to the inner conductor of the RCA connector.
12. Plus the keying circuit board into the J13 header socket on the RF board. Connect the molex plug to the molex receptor on the keying circuit board.
And that’s it. Plug one end of the RCA-RCA cable into the new RCA connector on the back on your K2, and plug the other end of the cable into the “PA KEY” RCA socket on the back of the KXPA100. Connect up a patch cord between the K2 ANT OUT and the KXPA100 RF IN, and you are good to go.
Here are some pics of the completed circuit, from above and from beneath.
When someone mentions “Beverage Antenna”, if you think of towers of
beer cans or picture fountains of coca-cola, then you need to come to
this month’s lecture at the Manly Warringah Radio Society.
Find out what a beverage antenna really is, why you need one, how it
works, and how to make one yourself. 8pm Wednesday 16 April at the
MWRS club house.
The slides from the lecture are available in Slideshare.
On Wed 19th Feb Nick VK2DX gave a technical lecture on the 630m band (472-479kHz). His lecture covered suitable transmit and receive antenna options (they are different!), as well as hardware and software for scanning the band visually. He also spoke of propagation characteristics, usually this band propagates via ground wave with a path loss advantage over higher bands however sky wave propagation can and does happen!
On Wed 15 January 2014, Matt VK2RQ gave a lecture on packet radio, which provided an overview of what is packet radio and how it works, and described some of the applications for which packet radio is used. Here is a link to the presentation slides in Slideshare. A video recording of the lecture is available here.
MWRS operated portable during the Sunday morning segment of this year’s Summer Field Day, from the oval behind our club rooms. This oval is at a high point in Terrey Hills and provides a good outlook in all directions. Operators were Nick VK2FS, Geoff VK2MIA, Matt VK2RQ and Carlo VK2MXC. In addition we had several club members turn up to say g’day and see how we were going.
Weather was warm and sunny, quite humid as it had rained overnight and the residual moisture was evaporating out of the ground.
For 6m, we used a squid pole supporting an inverted V, with the peak about 7 metres above ground level. FT-897D radio running on 2 x 12V 7Ah SLA batteries.
For 2m, 70cm and 23cm we used high gain Yagi antennas on a portable mast, rotated by hand. Unfortunately the coaxial relay for our 23cm equipment was playing up so we didn’t make any contacts on that band. IC-7000 radio running on large blue SLA battery.
Total contacts 19, with a great 6m opening to VK5, VK4 and ZL. Due to the nice opening, our score for 6m was almost our best category!