Tag Archives: uhf

Preventing the “power spike”

This problem was discussed on the moon-net list in May-June 2016.  I thought it was worth documenting what is going on with many radios that produce a “power spike”.

The problem is that the gain of the amplifier chain (from say, final mixer to output) will vary from rig to rig, also the drive level will vary. Therefore the actual gain required of the amplifier chain varies from rig to rig, purely due to component variations and even due to alignment settings, which are probably done fairly quickly at the factory.

The use of ALC to control the gain of the amplifier chain is a typical and common approach taken by manufacturers. It is a technique that does not work very well for modes other than FM and where external amplifiers are used, where the output power required is less than the maximum rated power of the radio (actually, less than about 1.5 X the rated power).

Other manufacturers also have these problems. They address them in different ways.

In one example, the Yaesu FT8*7 series, there are two controls for each band set (HF, VHF low, VHF high and UHF), being drive gain and output power limit.

Another example is Icom’s IC910 where I understand that the power level control has a dual action, one is to reduce the amplifier gain and the other is to change the output power limit level. So it achieves the same kind of result as the Yaesu 8*7 series, and should result in no power spike when first transmitting.

In any radio, if the driver gain is too high, the output power may momentarily exceed the preset level intended from the radio and set by the power level preset feeding the ALC circuit. The time constant of the ALC circuit determines the attack and delay times but cannot prevent the power level rising above the preset value, momentarily, and that’s all we need to exceed the limits of a solid state amplifier device.

So in the 8*7 series you can set the gain level appropriately so that the radio cannot output any more than your chosen power level, and it is a matter of alignment procedure to adjust output limit and stage gain appropriately to get the result you want. They do (cleverly) offer three power levels and you can set the gain and the output power limit for each power level.

For a radio capable of 100w output it is never going to be enough to set the output limit (driving the ALC) to (say) the 25w level. As already stated, that will still result in a power spike while the ALC sets the output level to what the user requires (via the output power control). It is more obvious and easiest to see in the constant carrier modes like fm and CW.

If the drive level is sufficient to allow the power amp chain to produce 100w, then the initial output (on say CW mode) will be 100w, and if you have set the output limit to 25w, feedback via the ALC circuit will reduce the power to 25w. But the initial spike will always be there. It may only last a few milliseconds, but with solid state circuits it is not a matter of heat or averages, it is whether the input voltage exceeds the correct level at all, for even the first sine wave at 144 MHz, ie. for 1/144 microseconds.

What produces the spike? There is enough drive to the final amplifier for 100w. The output limit setting may be set for a lower power level, if so the ALC line is used to send a gain control voltage back to the gain controlled stage(s).

To make it impossible for a power spike to be produced, the drive level has to be reduced. We need to limit the drive to the power amplifier chain to whatever is needed to produce the nominal power level, whatever it is, 1w, 25 or 75. This could be done in several ways.

The first and typical way to reduce the drive level, when using audio source and ssb mode, as for WSJT and other AFSK type modes, is to reduce the audio level going into the radio.  This would work, but if the reduction in drive required is significant (more than say, 10 db) that decreases the signal level without reducing the level of noise and other inevitable spurious signals, including the suppressed carrier of the ssb signal.  Eg a carrier suppression level of 45 db may be specified by the manufacturer, referenced to its performance at full rated power.  By using the audio drive to reduce total output we are accepting that the suppressed carrier can remain at its current level, and that may be ok for some radios.

The second method of reducing the output is to use a high power attenuator between the radio and the external amplifier.  This attenuator would be in that circuit on receive mode too.  For EME use many operators using the separate receiver antenna input to the receiver, or use another receiver anyway.  But the impact of the attenuator on receive mode is another factor to consider.

Other options include:

  • Modify the radio’s internal gain in the transmitter chain, preferably in the section amplifying at the transmitting frequency.  Depending on the design of the radio, there may be a point where the final mixer output is fed to the amplifier chain, which would be a good place to insert a suitable attenuator.
  • Insert a voltage on the ALC line, setting the gain of the transmitter to the highest it is allowed to be – reducing the output level to the highest it can be to safely drive the external amplifier.
  • Bypass amplifier stages.

Some types of mods would render the radio incapable of higher power output, so would need to be reversed when moving the radio on to another purpose. Whatever method is chosen, it must prevent the drive chain from producing enough power to drive the output stage to full power.

Importantly, it should be done in a way that is absolutely foolproof.  A casual mistake by the operator that destroys the external amplifier is something to prevent entirely.

The metering on the radio would be meaningless if some of these options were taken.  Separate methods of metering the drive level and adjusting for best operation would be required.

This is not a plug and play application. We are using a radio in a way that is outside its designed purpose.

The inability of the TS2000 (or other radio) to be used without modification for lower power purposes is no reflection of its suitability for other purposes. All commercial radios are built for the most common use by the majority of buyers. When we apply these general purpose radios to special uses such as for EME amplifier drivers, we cannot really be surprised that they are not ideal for EME drivers “out of the box”.

John Moyle Memorial National Field Day contest March 2015

As in past years I operated in this event at Mt Ginini in two ways.  On VHF/UHF bands I used my standard equipment powered by a Honda EU20i generator, with 100w output on 2m/6m, 75w on 70cm and 10w on 23cm.  On HF bands I ran 10w from battery power, to be SOTA compliant.

I started the site setup at 5pm Friday night, setting up the tent and the HF antennas.  Two squid poles supported these antennas.  One was a linked dipole for the HF bands from 40 to 10m.  The other antenna was a quarter wave vertical with elevated radials for 20m.

HF dipole
HF dipole
The vertical antenna for 20m uses this junction box as the termination of the coaxial feedline, the vertical element above it and three radials each a quarter wave long attached to the binding posts on the sides of the plastic box. This is usually carried up to SOTA summits so needs to be light.


The VHF/UHF antennas were erected on Saturday morning.  Matt VK1MA and Glen VK1XX arrived to perform some maintenance work on the tower for the repeaters run by the Canberra Region Amateur Radio Club.  When I was ready to lift my antennas they were ready to help and fortunately I only needed to adjust the guy ropes.

VHF/UHF antennas being assembled prior to attaching feedlines
VHF/UHF antennas being assembled prior to attaching feedlines
VHF/UHF antennas being assembled prior to attaching feedlines
VHF/UHF antennas being assembled prior to attaching feedlines

On VHF the band conditions seemed ok, with the VK3RGL beacons on 144.530 and 432.530 were both received with reasonable signals.  Towards Sydney the beacons on 144.420 and 432.420 were weak but detectable.  Propagation in the north east direction (Sydney and up the NSW coastline roughly) remained ordinary for the weekend.

By the late afternoon, I had logged a small number of contacts on 40m and on the VHF/UHF bands.  There were a few other field stations, the most prominent on VHF being VK3ER and VK3KQ and I could work both on 6m/2m/70cm without much trouble.  The 23cm signals were detectable but only workable on peaks of the fading always present on that band.

In the hour before sunset I was working some 20m CW contacts as a SOTA portable, conditions did not seem too good on 20m towards Europe but I made a handful of contacts with Europeans and some Australians.  The planned ssb activations in Europe were basically inaudible, though with some imagination I could hear faint voices and stations calling them.  When you cannot really hear the chasers you know it will be hard to work the activators.

Returning to the VHF/UHF bands I had some good contacts into the area west of Melbourne, then heard VK5SR in the Mount Gambier area with a big signal.  Contacts with VK5SR were made on 144 and 432, but no signals heard on 1296.  Contacts were made at much increased signal levels with VK3KQ and VK3ER on 1296 as well as the three lower bands.  VK5RX was worked also on 144, a much more westerly contact in the PF95 grid.

I made a recording of an hour of the vhfuhf contacts on Saturday evening and it is published on dropbox.  The link is https://www.dropbox.com/sh/nk1raqnjobsi05q/AAC0xA2I82UVYjDQvD_gxpo2a?dl=0 and when listening to the signals from vk5sr on 144 and 432, remember that those signals are from a station 777 km away. VK3KQ was at a distance of about 500 km. 

During the recording you will hear a contact with vk3er on 1296 where they were so strong with their dish on my direction that I thought they were a local. Then I could hear vk3er at a distance of 460 odd km on 1296 even while they were beaming to Mt Gambier with their their dish 120 degrees off my direction. Conditions were unusually good! Following the contacts with vk3er on 1296 and 50 mhz there was a contact made on 1296 with vk3kq after several tries using 432 for liaison. There was an unsuccessful attempt, another set of dits used as a beacon, then finally a successful contact on ssb.  

By about 10pm the wind had increased in strength and it seemed unlikely there would be any new contacts made. I didn’t plan to operate after midnight to make contacts in the next 3 hour period so I closed down for the night, lowering both antenna masts so as to protect the antennas from the wind.  Having seen stakes almost completely ripped out of the rocky ground by gusty winds in past events, I didn’t want to risk damage to the antennas, the tent or the operator!

I woke at about 5am and was very cold, having packed the wrong sleeping bag.  It was about 4C that morning which was an improvement over the 2C of Saturday morning, however I warmed up in the car for 20 mins before raising the antennas and getting the station back on the air.  A few field stations were ready for contacts prior to 6am but despite trying to work them all before 6 on all bands, a few contacts were missed.  Due to the 3 hour time blocks used in this contest it is possible to make contacts in each 3 hour time block, at any time.  After the initial flurry of contacts with VK3ER VK3KQ and VK2WG it was time to check the beacons especially looking for VK5 beacons given the good conditions into VK5 the night before.  Some of the VK3 beacons were audible, the VK3RGL were good signals on 144 and 432 but the Mt Gambier VK5RSE beacon was not heard.  However the VK5VF beacon close to Adelaide was a good signal so I started making CQ calls beaming to Adelaide on 144.150.  During the next few hours several VK5 contacts were made on 144 and 432, with a marginal contact made on 1296 with VK5PJ.  Jeff VK5GF joined in the fun and his signal remained good for  several hours.  The VK5 signals were still good after 9AM so we were able to make several contacts for the field day log at these excellent signal levels.

Near Wagga the VK2WG club station was also making contacts into VK5 on 144 and some on 432, though signal levels were markedly lower than those received at Mt Ginini’s altitude of just over  1700m.  John VK2YW was operating the VHF station there and he has since commented that he wants to get onto 1296 after hearing of the contacts made there.

Operating desk, transceivers, microphones, morse paddles, power supplies and rotator controls.

IMGP1472 IMGP1475 IMGP1477

Antennas for 144, 432 and 1296 MHz on the foreground mast, with the 3 element beam for 50 MHz on the mast to the rear.
Antennas for 144, 432 and 1296 MHz on the foreground mast, with the 3 element beam for 50 MHz on the mast to the rear.

I think this event was my most successful field day from a VHF/UHF perspective.  The conditions on 144 and 432 were above their usual level but the results on 1296 were my best ever.   The vhf and uhf bands are a lot of fun in these great conditions.

VHF/UHF Field day January 2014

I set up my usual station on Mt Ginini QF44JL for this event.

On 50 MHz, a TS670 and a HL66V amplifier producing 60w to a 3 el cushcraft yagi on a 4m mast.

On 144 MHz, the IC910H 100w to an 8 el yagi at 6m agl.  On 432 MHz the IC910H 75w to a 16 el yagi at 4.5m agl with an icom mast head preamp 1.5m from the feedpoint.

On 1296 Mhz I had unfortunately not packed the pair of 18el yagis normally used.  As a token antenna to make some local contacts, I connected a 2m quarter wave vertical with about 3m of RG58 coax and laid that horizontally on the roof of the tent, bisecting the side and centre aluminium stressors that are part of the roof structure of my old Coleman tent.  This “antenna” gave me some local contacts on 1296 and with effort, a contact with vk2smc near Nimmitabel.

On Saturday I found conditions ordinary with no unusual contacts made.  On Sunday morning at 5AM local time I checked the usual beacons from Sydney, Mt Anakie in VK3, Mildura in VK3 and the Gippsland beacon, on both 144 and 432 where possible.  With the very calm conditions overnight I wondered if I would hear any beacons from further afield and checked the Mt Gambier beacon VK5RSE on 144.550 and the Adelaide beacon VK5VF on 144.450.  Both beacons were received at good strength, and during the following 5 hours both beacons remained audible, the Adelaide beacon being the strongest signal most of the time until it faded around 9AM, the Mt Gambier signal remaining audible but weak for a little longer. My log notes that VK5RSE was still audible at 2305 UTC, or 10AM local time. At that stage the Adelaide signal had vanished.  With these beacon signals received so well, how about making some contacts into those areas?

I then worked Bill VK5ACY at 1922 UTC (6:22 local) vk5LA at 1939, vk5AKK at 2006, VK5PO at 2008, VK5DK at 2109, all on 144MHz.  I also worked Vk5AKK on 432.

Much later at 2150 I was called by VK5PJ on 2m while beaming to Sydney direction (NE) and made a good contact with Peter on that band, followed by working him again on 432 MHz, still with the beams NE.  Turning the beams around to the west produced signal levels of S9+20 (indicated) which is a rare event on 70cm dx.  Peter asked whether I had 23cm and I told him that regrettably my real antennas were at home and all I had was a temporary lashup to make local contacts.  He was keen to try it given the unusually good propagation we had on 70cm.  We tried 23cm first with Peter running a series of dots, so I tuned for that signal on the Sub receiver on the IC910 and could tell him “yes I do hear that, I will send the same to you”…  and the outcome was a good 5 x 1 contact on SSB.

Back on our “liaison frequency” 432.160 where signals were still s9+ I told Peter what the antenna was.  “It’s a 2m quarter wave lying on the roof of my tent”.  He asked for a photo…I took the following photo immediately while still sitting at the desk talking with him.

Note the precise calibration of the angles.

horizontal 9/4 wave antenna
horizontal 9/4 wave antenna

Here is what it looked like from the outside.

Tent roof antenna
Tent roof antenna. Note the centre boom and the supporting spines all aluminium. the antenna received best signal on 1296 when at 45 degrees to those tent supports.  Adelaide is close to the direction of the centre boom.

I then asked him to run the beeper again so I could try to optimise the orientation or location of the antenna.  I tried vertical and horizontal polarisation in various orientations.  Eventually I returned the antenna to its original position where by good luck, the signal was best.  You would not read about it.

Later at 2223 I was encouraged to give this antenna a try working VK3ER where Peter VK3QI was keen to make the contact.  And yes it did work, even on ssb.  In the past we have made contacts with my real antennas but sometimes it has been quite difficult, cw-only.  Clearly propagation was unusually good between us.

A later attempt to hear or work Gordon VK3EJ at Cobram was unsuccessful.  Whatever atmospheric effect was allowing these longer distant signals to reach Mt Ginini was not active for the shorter distance to Cobram.

This is where the 1296 yagis would normally go… just below the 70cm yagi on the mast.

2m and 70cm antennas.
2m and 70cm antennas.

Summary: 145 or so contacts, some ordinary and a small number of extraordinary contacts, coinciding with very hot daytime weather and a calm morning.

Once again the beacons were a great indicator of the possibilities ahead.

As my brother Chris VK2DO pointed out, it looks like the many tickets purchased in the “field day lottery” over the past 20 years have finally paid off and I have certainly been rewarded with some great fortune this time.  If only I had my real antennas for 1296, and how about the higher bands?  Will never know, can only continue to take tickets in the lottery and hope it doesn’t take another 20 years to produce results.

VK9NA expedition

Late comments about the VK9NA expedition I joined in January 2011.  This was a VHF/UHF/microwave and 2m EME operation.  Due to quite poor conditions for tropo across to the mainland, we eventually did most of the operation on 2m EME.  However we did try to make contacts and ran a lot of CQs on 144 MHz every day.  We did make some contacts but there were nowhere near the number of tropo contacts made last year.  The 144 MHz band was the main band used for this work.

We activated the station every day on 6m as well, from the hotel site.

Due to the high winds experienced on the hill we moved the EME station to the Guide Hall where we had been kindly offered the use of the grounds.

On Norfolk the internet access is provided by Wifi connections at hotels/resorts and a few in the Burnt Pine business area. I found it was necessary to buy several different cards to get access via NIDS, Norfolk telecom and another account for access at the hotel I stayed at. Wifi access from Mt Pitt was good, from the hotel the others stayed at, access to NIDS was not good.

The radio conditions on vhf up were not as good as they had been in 2010. This was partly due to physical weather conditions, including strong winds for the duration of the operation from 8th to 20th January. On the weekend of the summer field day conditions were very poor and the only contacts made with the mainland that weekend were on 6m, and there were not many of them.

The 2m EME operation was very successful.  Over a hundred contacts were made using JT65 via the FT897 and a laptop computer running the WSJT software.  A TE systems amplifier boosted the output power of the FT897 for EME work.  The list of stations worked is at the VK9NA website.

I greatly enjoyed the event.  I learned how to use WSJT on both FSK441 and JT65B, and learned a bit about pointing a very large 2m antenna (19 elements, 12 metres length) at the moon and periodically repointing it. For about half or more of the time, the moon was not visible so we were relying on compass bearings corrected for mag offset/declination and an inclinometer for the elevation.

I also became familiar with the FT897 and found what a great radio it is for this kind of operation. The other radios used were FT817 and a TS2000 which I found to be a very good radio too.

The TS2000 has an option to automatically transmit CW at a 700 hz offset (actually the offset equals your selected cw beat note and sidetone frequency) when you switch from USB to CW. It also has an option to automatically switch from SSB to CW mode if you press the key, whether it’s an automatic key or a hand key. Very neat.

Apart from the radio aspects it was also great to get to know Michael VK3KH, Alan VK3XPD, Kevin VK4UH. We were fortunate in being well organised on the social and meals front by Michael’s wife Roz and her sister Gail, and Alan’s wife Aileen all of whom made this event that much more enjoyable.

We did attend a few local special events such as the fish fry, the progressive dinner and the re-enactment drama based on the voyage of the Bounty, the eventual mutiny led by Fletcher Christian and the exile of the mutineers at Pitcairn Island. This history is a proud aspect of the Norfolk Island culture today.

A great event and a fun filled 10 day trip for me.

Here are some photos at Flickr:

Here is one photo of the EME antenna.  Remember  it is 12 metres long.  There are 19 elements.  Click the photo for a larger view. The long “element” in the centre of the boom is just a truss boom – the antenna has vertical and horizontal stabilisation to prevent it flexing and losing gain. EME antenna at VK9NA

VK9NA photo gallery

Gallery of photos from the VK9NA Dxpedition in 2011. Click any photo to view larger size, with slideshow control options.



Joining VK9NA VHF/UHF dxpedition to Norfolk Island

I am joining the VK9NA team for January 2011. All the details of this expedition are on the VK9NA.COM website. This is a VHF/UHF/microwave expedition which will include some 2m EME capability and will have reasonable power (75w) on 5.7 and 10 GHz too.

The station should be on the air by 9th January and will be active in the following weekend’s VK VHF/UHF Summer Field Day event.

More details on the VK9NA website vk9na.com

VHF contests: points for distance or for grid squares?

At the Dayton Hamvention 2010, I attended the VHF weak signal group dinner.

I met and chatted with a number of other people about VHF activities in Australia and heard discussions on contest rules that were familiar issues.  Should contest points be based on distance or on grid squares, or power, or what?

In the US VHF sprints they are trying a distance based formula based on 6 character grid locators.  They have found that this approach has been well accepted by contest participants.  It is now quite feasible to calculate distances based on 6 character locators, since computers are so common.  Maybe this is what Australian VHF operators would like.  The grid square bonus system is much simpler but some people think it doesn’t give recognition or incentives for longer distance contacts.

It will be interesting to see whether they decide to keep the distance based scoring formula.