The HF antenna…

So, most members should be aware by now that the old 40m dipole has been removed and replaced with a new 40m dipole!  The feedpoint 1:1 balun was also changed for good measure and sealed thoroughly for the winter.

But this is not the end of the work on the HF antenna, that was just task 1 😉  Task 2 consists of two activities and is planned for Spring 2017 once it dries and warms a little 🙂

Activity 1 will add elements for 30m and 20m expanding the 40m dipole into three parallel dipoles. This will result in a tri-band antenna that is resonant on 40m, 30m and 20m giving us a very useful antenna.

Activity 2 will replace the coaxial feeder between the radio room and the antenna feedpoint.

TSGARC parallel dipoleToday we have approximately 2.3dB of loss at 7.1MHz between the radio room and the antenna feedpoint.  Assuming the coax is in the very best condition (which its not) and it’s RG-58C (Its an RG58 of some sort) this equates to about 54m of coax.  That’s a 40% power loss on Tx and Rx.  Replacing this with RG-213 reduces the loss to 0.9dB or a 19% power loss.  An improvement of 1.4dB or 20% doesn’t sound much and its not but consider the situation at 14MHz.  With RG-58C there is 3.2dB / 53% loss but using RG213 the loss is 1.3dB or 26%.

Frequency RG-58C RG-213/U Ultraflex-10
7.1MHz 2.2dB 0.9dB 0.5dB
10MHz 2.7dB 1.1dB 0.6dB
14.2MHz 3.2dB 1.3dB 0.8dB

Coax loss per 54m 

To put this differently.  The transmitter places 100W max into the feeder at 7MHz, 10MHz and 14MHz.  With RG-58 the antenna receives: 60W, 54W & 47W respectively.  Replacing with RG-213 results in: 81W, 77W & 74W.  Using a better coax and one that does not cost  much more than RG-213 the situation can be improved further.  The overall difference of using something like Ultraflex-10 over RG-58 is definitely worth having and that is why activity 2 is so important.

The completed parallel dipole antenna will remain in its current location supported by the trees at either end.  But supporting the end of the antenna elements will require two attachment points instead of the one we have today.  Investigation has showed this is not a significant obstacle.

Thanks to Peter, G3LDO for the above diagram taken from his excellent book Backyard Antennas.

Antenna Maintanance

Yesterday myself, Andrew, Graham, Rex, John and John spent the day at the Chantry renovating the club’s antenna systems.

Our plan for the day was to inspect the existing antenna systems, replace a section of coax between the attic and balun and to install and tune the new 40m dipole.

Condition of existing antenna systems

Existing 40m dipole balun

Existing 40m dipole balun

The overall condition of the existing HF antenna was good, but clearly the self-amalgamating tape has suffered from UV degradation. This doesn’t appear to be a major problem however the antenna has only been up roughly 4 years, so the problem would get worse over time.

A photo of the existing co-linear VHF antenna

Existing VHF Co-linear

While on the roof we also inspected the existing VHF antenna. It is clear that it is not ideally placed, being blocked by the chimney stack as well as the Chantry’s satellite dish.

Replacing the coaxial cable

One of the jobs for the day was to replace the RG-58 coax run between the attic space and the existing balun with some superior Westflex 103. In order to do this, Andrew and myself had to climb into the attic in order to cut the existing cable and solder new UHF connectors onto it. This allowed us to replace the cable between the attic and the antenna. On completion we measured the loss through the cable using a 50Ω dummy load and power meter.

new cable run

New Westflex 103 cable run

We measured the loss at just under 3dB at 7mHz. We measured 60W at the end of the cable with a 100W input. Currently we plan to improve this loss further by replacing the rest of the cable to the shack

Andrew holding a dummy load and power meter

Andrew getting ready to test the loss through the new cable

Installing and tuning the new 40m dipole

With the new coax installed, we now set to work installing the new antenna. Before we could putt the new dipole in place we had to let down the existing antenna. As the two ends were attached to trees, I volunteered to put my tree-climbing skills to use and lower the existing elements

Rex at the foot of a ladder

Rex giving us a hand getting up the trees

With the antenna on the ground we attached the new dipole and hoisted it into the air. Following some tests with Andrew’s MFJ antenna analyser we deduced that the elements were too long. Following several rounds of tuning we reached a state where everyone was happy with the performance of the antenna.

New 40m dipole balun

New balun installed

We sealed the connections with self amalgamating tape and headed to the shack to have a listen to the band.

40m dipole

 

Performance

Frequency (mHz) Resistance (Ω) Reactance (Ω) SWR
7.0 60 8 1.2
7.1 78 0
7.2 112 0 1.5

We also tested the SWR in the shack and measured an SWR of 1.4 at 7.2mHz. A quick sweep of the band picked up some loud and clear Morse, although this may be due to a contest that was running at the time.

Conclusion

We hope that the new 40m dipole continues to deliver excellent results. There are still improvements that need to be made to the club’s antenna systems, including:

  • Adding the two other elements (30m and 20m) to the fan dipole array
  • Replacing the coax run between the attic and the shack with lower loss cable
  • Deciding on an effective way to relocate and improve the VHF antenna system.

I’d like to thank Andrew G0RVM, Graham, Rex G4RAE, John M3EQQ and John M0HFH for giving up their time yesterday to help the club. I had a great day and learned a lot throughout the process.

Peter Barnes
2E0UAR

RSGB Convention 2016 update

Camb-hams FlossieMyself G0RVM and Peter 2E0UAR attended the RSGB Convention in Milton Keynes this year.  We left Bristol around 15:30hrs but got stuck in jam after jam.  Towards the end of the journey ‘here comes another set of blue lights’ was becoming a bit of a joke!  Fortunately we did arrive before the buffet dinner finished.  It was a close thing tho as there where only a few slices of pudding pie left 😉

This was my first Convention and I must congratulate the RSGB for such a great event.  The accommodation was good, the food and conference facilities excellent.  It was educational to hear talks on a variety of subjects from speakers deeply knowledgeable in their subject.  Access was free to those under 21yrs too – a great way of incentivising attendance by younger radio amateurs.  Thank you RSGB.

In addition to the rooms hosting five parallel lecture streams there was a room with stands by Icom, Kenwood, the RSGB and of course, Martin Lynch who were the prime sponsor.    Outside this room was a rather large trailer tower with HF antennas providing live signals for the exhibitors.

Kenwood TS-990Proudly parked outside the front of the conference centre was Flossie, the mobile radio van of Camb-hams.  Protruding through its roof was a Clark pneumatic mast with rotary HF dipole.  The van looked excellent and a great way to get a portable setup to a distant location, setup and on-air with minimum fuss.  As I have a pneumatic mast also it was good to swap experiences, finding we shared some of the challenges associated to these masts.

Some of my favourite talks over the weekend were:

    • The new world of amateur satellites, Graham Shirville, G3VZV
    • The Story of SDR and FlexRadio, Gerald Youngblood, K5SDR
    • The VP8SGI & VP8STI DXpeditions, Mike McGirr K9AJ
    • Space Weather, Prof. Cathryn Mitchell, M0IBG.

I was really looking forward to “Best practice for VHF UHF DX” by Ian White GM3SEK but unfortunately Ian had to cancel.  Maybe next year.

Camb-hams FlossieFrom the first talk identified above I learnt that we as radio amateurs are soon to have a ‘bend-pipe’ transponder in geostationary Earth orbit.  Wow that is an amazing feat and I can’t wait till its operational.  Amazing.  The talk by Prof. Cathryn Mitchell was excellent being delivered superbly and hugely informative.  It was interesting to hear how in 2015 space weather was identified in the UK National Risk Register with an impact of the same scoring as emerging infectious diseases, inland flooding, effusive volcanic eruptions, major industrial accidents etc.  In fact, in 2015 the likelihood of a major space weather event occurring in the next five years was in the second from highest category.

I can thoroughly recommend attending the Convention and I know I will be booking my 2017 ticket as soon as they become available next year.

Andrew
G0RVM

Fox Hunting with a TDOA Antenna

In preparation for the pedestrian fox-hunt that happened a couple of weeks ago, I decided to take a look around for some antenna designs that I could build at home. I came across a blog post which showed off a Time Difference of Arrival (TDOA) antenna that someone had designed and built.

TDOA Antenna

A simple and effective fox-hunting antenna

How it works

The antenna relies on a timer, in my case a 555 timer, to generate an audible signal. I have mine calibrated to a frequency of 1kHz. This signal is level-shifted so that it goes between -4.5V and 4.5V. The use of a set of diodes causes the circuit to quickly switch between the two dipole antennas mounted to the unit (tuned to 145mHz). When the signal received by the two dipoles is out of phase, the receiver emits a tone. When the RF source is equidistant from both antennas (when you are pointing at the “fox”) the signals are in phase, causing the tone to disappear.

This design is fully functional regardless of how close you are to the source, providing an advantage over other antenna designs. The audible tone also provides a clear indicator of direction. However, the main issue is the fact that you get two readings, at 180° to each other.

Parts List

The list of components required is quite simple, so I had most of them laying around.

  • 555 Timer
  • 4.7K  Resistor
  • 100K Resistor
  • 2x 470Ω Resistor
  • 2x 0.01uF, 50V Capacitor
  • 10uF, 25V Capacitor
  • 0.001uF, 50V Capacitor
  • 4x 1N4007 RF Pin Diode
  • 2x 10uH Inductor Coil
  • RF Choke, 8 turns (salvaged from a motherboard)
  • SPST Switch
  • 9V Battery
  • 9V Battery Clip
  • Antenna wire (single core) or 4x Telescopic Antennas
  • 1.5M of RG-58 Coax

Build

When building the antenna, I used two blog posts for reference. You can find them here(pdf) and here. For some reason I ended up combining both of the designs, using the Bryonics.com design for the dipole circuits, and the 146970.com design for the rest of the circuit. This probably isn’t advisable, but it worked for me. Here is the circuit I used:

You can test the 555 timer circuit by hooking the output pin up to a speaker. You should hear a tone when power is supplied to the circuit. I started by breadboarding the timer circuit, and then moved it to veroboard for the final build. Once the circuit was complete, I zip tied the PTT button down on my PMR446 Walkie-Talkie, plugged the antenna into my Wouxun KG-UVD1P reciever, and tuned into the 446mHz signal. The antenna worked perfectly first time, providing a clear tone that faded out when I pointed the antenna at the radio.

For the physical construction of the antenna, I used some thick, single core, cable to make the dipole antennas, and mounted the whole thing on a 60cm long piece of wood. Lots of glue gun was used as I couldn’t be bothered to find some screws. I also drilled holes in the ends of the wood to poke the coax through. The dipole elements were attached using screws, and can be bent between a folded and deployed position.

CircuitEvaluation

I took the antenna out for the pedestrian fox-hunt that happened a few weeks ago. The antenna picked up the first signal very well, and the audio tone allowed me to find the direction of the transmission to quite a high accuracy. Unfortunately for me, I headed off into the wrong direction, as the antenna will give you two readings at 180° to each other. I had it running all evening, without the battery flattening. The RF choke fell off quite quickly, as I had not done a very good job with the soldering. I was also worried that the connection between the RX input coax and the board would break, so I would consider using a small connector in the future (such as an SMA socket). Overall the antenna worked very well, albeit sending us in the wrong direction! In terms of improvements, I would also like to mount the circuit board properly, and put it in an enclosure.

If anyone has any questions, or wants me to bring mine with me on a club night, just let me know.

-Peter Barnes

Cobwebb resonating. Part 2

Last week I wrote about recent work that identified where my G3TPW Cobwebb antenna was resonating on each of its five bands (20m, 17m, 15m, 12m and 10m).  Following on from that work G0MGM and myself spent a day recently adjusting my Cobwebb so that it was resonant around the SSB sections of each band and this article summarises that work.

The instructions supplied by G3TPW for his Cobwebb are excellent and identify the tuning effect of shortening/lengthening each dipole leg.  For reference I have identified these below:

Band Change
 20m  40kHz/cm
 17m  50kHz/cm
 15m  75kHz/cm
 12m  100kHz/cm
 10m  120kHz/cm

What the instructions omit is whether the dipoles interact, whether they should be adjusted in any sequence (e.g. 20m before 17m) and the impact of extending/reducing the gap between each dipole leg (spanned by the string) upon resonance.  These were all questions that were going through my mind prior to starting adjustment work and which drove the approach adopted.

The reactance, resistance and impedance data presented in Part 1 was collected using my MFJ-259b antenna analyser.  The MFJ-259b is a basic analyser and does not have any capability for data logging or data export necessitating the collection of data at multiple manually sampled frequencies.  This process was laborious and constrained the number of samples it was practical to collect and thus the accuracy of the overall result.  As I foresaw the need to resample each of the five bands for each single adjustment, it clearly, was not going to be practical to use the MFJ analyser.  Fortunately, a good friend, G0MGM, has a miniVNA analyser that can auto-sweep a band, log the results and export them in a CSV formatted file, which we later imported into Microsoft Excel. This capability made it practical to capture data samples, visualise and analyse the impact on each band of every change.

All graphs in this article may be enlarged by ‘clicking’ upon them.

Baseline

Because a different analyser was used, two new sets of baseline data were captured with the antenna at 3m and 8.5m above ground.  The lower height represents the  height of the antenna when my mast is retracted.  The baseline data presented in this article is that sampled at 8.5m.  Measurements were taken in the radio room at the end of the RG-213 coax feeding the antenna.

20m (Baseline)17m (Baseline)15m (Baseline)12m (Baseline)10m (Baseline)

It is interesting comparing the baseline results above with those captured previously using the MFJ analyser and presented in Part 1.  It should be noted that the comparison was performed with the antenna at the same height, with the same coaxial feeder, but on different days, that the weather was similar and that on both occasions the antenna and its surroundings were completely dry.  Furthermore it should be noted scales and colours vary thus some interpretation is required.

Adjustment

Based on experience of adjusting a Butternut HF-6V antenna, now made by DX Engineering, and a need to start somewhere, the decision was made to sequence adjustments from 20m, progressing to 10m.

After analysing baseline results it was decided to adjust the 20m, 17m and 15m elements.  Starting with the 20m element we reduced the length of each leg by 2cm and resampled data across each of the five bands.  Results showed that the change had a positive impact, raising the 20m resonant frequency by the amount expected with little or no change on any of the other bands.  This was good news and was the first indication that there was little interaction between the five elements. We then repeated the process removing a further 2cm from each leg.  Again the results were the same. i.e. the change on 20m was that expected and there had been little or no change on the others.  The graph below is the final result of the two changes.

20m band (Final)

Next the 17m dipole element was adjusted, reducing each leg by 2cm.  The process of sampling across all five bands was repeated and again it was found that the change had no significant effect upon the frequency of resonance for the other bands.  This really gave confidence that each element could be adjusted independently and that no sequence of adjustment was necessary.

Only one change was necessary and it raised the resonance point to the frequency required.

17m Band (Final)

Sticking with the original plan, although it was almost certain by now, the 15m element was adjusted reducing each leg by 1cm.  This raised the resonant frequency to that required.  This time some minor changes were noted to the point of resonance on the other bands, however, the change was very small.

The changes to 20m, 17m and 15m had necessitated the retying of the string between there individual leg elements.  The string between the unchanged elements had remained unchanged and it was now observed that there was noticeably more slack in the wire at the leg ends for those elements.  It was thought that this additional slack may be causing the very small changes observed.

15m Band (Final)

With changes complete the results from sampling each of the five bands were analysed and found to be acceptable so again the antenna was lowered and the length of string on the unchanged elements reduced so as to tighten the wires slightly.  The change in string length was small but afterwards it was noted the resonant frequency had raised a little on those band elements.  This was expected as reducing the gap between each leg end adds capacitance.

The results for 12m and 10m are shown below.

12m Band (Final)10m Band (Final)

Conclusion

Adjusting the Cobwebb proved to be much simpler than expected.  Results showed that each dipole element could be adjusted without impacting other elements and that adjustment need not take place in any particular sequence.  It was also found that frequency change per cm as specified by G3TPW in his instructions was accurate.

In writing this article it was realised that it would have been useful to capture the length between the ends of each element leg.  i.e. the string length.  When these can next be measured I will update this article with the information.  Describing the tautness of the elements is difficult.  They are neither taut nor slack, but ‘just right’.  i.e. there is a little movement of the wire.  Perhaps it is better to describe by stating that their tautness does not deform the cross shape of the fiberglass spreading arms.

Key to the success of the adjustment work was the miniVNA analyser and its ability to visualise and log sampled data.  Without it, what took approximetly 5hrs would have taken much longer.

Finally, thanks to Rob, G0MGM, for his assistance and his miniVNA and enjoy the bottle of sake 😉

Andrew
G0RVM

Cobwebb resonating. Part 1

cobwebbI’ve had a G3TPW Cobwebb for almost a year but recently its moved location and is now on the top of a Total Mast Solutions 11m pneumatic mast.  Because previously it was in a temporary installation I didn’t think much about ensuring that resonance on its five HF bands (20m, 17m, 15m, 12m & 10m) was where I wanted it to be.  I just used a manual coupler (aka ATU) to ensure its match to the transceiver was close to 50 Ohms.

The antenna has been performing very well considering its very small size and has yielded worldwide contacts but I decided recently it was time to do some investigative work to see just where it was resonating and thus whether it could be further improved.  I have an MFJ-259b antenna analyser which lets me identify the resistive and reactive components at a given frequency.  However, and annoyingly, it doesn’t support any sort of automated band sweep or result logging capability.  Therefore its necessary to take and record multiple individual measurements then manually enter these into a graphing tool (MS-Excel in my case) to visualise the results.

To provide a reference baseline I captured the resistive and reactive components both with the antenna at 3m and 10m above ground.  As the results were similar I’ve included only those results when at the greater height and these are shown below.

Select each for a larger size.

20m 17m 15m 12m 10m

In the coming days a good friend G0MGM who has a MiniVNA analyser has agreed to help.  His analyser has two of the key capabilities my MFJ-259b lacks: Band sweep and result logging.  The intention is to adjust each dipole individually to achieve resonance just where I want it.  However, I’m not clear how the five dipole’s interact, I assume they must to some degree as they are closely spaced, or whether its best to adjust them in any sequence.  i.e. 20m before 17m.

In part 2 of this post I hope to report what we found, the adjustment methodology and importantly the results.

Andrew
G0RVM

Linear loaded 40m dipole

Whilst investigating options for a new 20m/40m antenna at my station I came across a QST magazine article by Lew Gordon, K4VX about a linear loaded dipole for 40m.  It’s not dual band, but it did catch my eye due to the well written article and also because of its shorter, than a full size dipole, top length.

I’m posting it here just in-case it’s useful to someone.

Andrew
G0RVM

The Dipole Antenna

There is a good introductory segment by Gordon West, WB6NOA to the dipole antenna, the benefits of a resonant vs non-resonant antenna and a discussion of other simple wire antenna’s near the start of the the latest Ham Nation episode.  We have a link to all episodes under ‘H’ on our Links page but for convenience you can also find it here: http://twit.tv/show/ham-nation/162

Ham Nation is in its 162 weekly episode and since the very first episode they have covered a wealth of subjects core to amateur radio.  Memorable episodes for me are Bob’s introduction to microphones, audio processing and the importance of good audio to working DX, coaxial dipoles, the station ground (RF vs DC) including ground loops plus some of the kits George has built on the show, but there are many more.

If you don’t know about Ham Nation, I can certainly recommend viewing.  It’s an all American production by Bob Heil (K9EID) [Of microphone fame], Gordon West (WB6NOA)George Thomas (W5JDX) and Dale Puckett (K0HYD) and episodes often contain contributions from the UK.  Its very good and has quite a worldwide following.

Andrew
G0RVM