Amplitude Modulation (AM)

At one time this was the dominant mode on all bands. With more stations and the advent of SSB, AM was widely scorned on HF as it took too much space and was not as effective or efficient as SSB. Actually there is plenty of space for AM on some bands.

On 10m there is plenty of space, with 29-29.1MHz (someone please tell the RSGB) as the best place to find AM. There are AM nets on 160m and 80m and there is plenty of space on VHF. For some reason AM is "out of fashion" on the VHF bands with FM the predominant mode. Certainly in the UK you can often get AM gear (ex Private Mobile Radio) at knock down prices. On 2m, AM activity can be found around 144.55MHz in the UK (again someone please tell the RSGB that AM usually takes less bandwidth than FM).

So, overall, AM is worth a go. Personally I would not use AM on busy HF bands, but there are bands where it makes good sense. I well remember not many years ago hearing beautiful AM from the USA and Canada. on 10m AM.

See www.amrally.com .
See https://sites.google.com/site/g3xbmqrp3/hf/tenbox

RSGB, bandplans and AM

What is it about the RSGB and AM?

Yet again, AM is reduced to a 2m footnote with a stupid, childish and inane remark about watching bandwidth! OK, it is some time since I used AM seriously, but the RSGB really needs to wake up. AM bandwidth is less than many FM transmissions and very low cost AM gear is available from the PMR industry.

I am rapidly going off the RSGB. The image of a bunch of grey haired fuddy duddies who are brain dead is being heavily re-enforced. I am sure this is not the image they want us to see.

As a member of the RSGB for 54 years, it is possibly my last. Although I want to support my national society, I find it hard if they are incompetent.

Heathkit Twoer and Sixer transceivers

If, like me, you are rather old you may remember the Heathkit Twoer, Sixer and similar rigs. These were 5W AM transceivers covering either 10m, 6m or 2m. I think there was also one covering 11m for the early USA CB market.

These were single channel transmitters with a super-regen RX. Although I coveted one, the nearest I got was owning a manual!

In its day, it opened up the VHF bands to the masses, long before SSB and FM. I liked the style then and do so even now. To be honest, today they would struggle, as there is not much AM on VHF and the bandwidth of the RX would be too wide for serious use.

In 1966 it sold for £26 in the UK! On the second-hand market good ones command a good price, no doubt bought by nostalgic old timers!

See https://sites.google.com/site/g3xbmqrp3/vuhf/lunchbox

The 2m Fredbox

This was designed in 1974, when I was still "green behind the ears". It got rebuilt much later and still worked well.

The Sixbox came much later.

See https://sites.google.com/site/g3xbmqrp3/vuhf/fredbox .

Sixbox 6m AM QRP transceiver

As we slowly progress to "quiet sun" years, it is worth remembering that our VHF bands are useful for local QSOs. Some years ago I designed and built the Sixbox which was a very simple AM transceiver for 6m local communications. It should be easy to build and develop further - maybe with more power and a loudspeaker amp added. I am pretty sure this has been shown before.

See https://sites.google.com/site/g3xbmqrp3/vuhf/sixbox

AM in Portugal - Nov 5/6th

Southgate News reports on an AM weekend in Portugal in November:

"ARLA - The Portuguese 'Associação de Radioamadores do Litoral Alentejano' is organizing a whole weekend of Amplitude Modulation-AM on the 6th and 7th of November, the sixth edition of the event previously known as 'Dia Nacional do AM em Portugal' (National Day of AM in Portugal).
The broadcast, in amplitude modulation, will take place in 80m, 40m, 20m, 10m, 6m and 2m between 08AM UTC Saturday Nov 5 until 11:59PM UTC Sunday Nov 6.
Therefore, ARLA asks all ham operators and SWL, national or worldwide, to participate in this special broadcast, widen this year and for the first time to two consecutive days, in AM and in the amateur band plan.
The broadcasting will take place in several frequencies and the following should be considered for calling or meeting points between the participants:
3 705.0 kHz (+- 5kHz);
7 146.0 kHz (+- 5 kHz);
14 270.0 kHz (+- 5 kHz);
29 075.0 kHz (+- 5 kHz);
51,575.0 MHz (+- 5 kHz);
144,550.0 MHz (+- 5 kHz).
As referred above, this year we've widen the event to 48h due to several requests of previous participants, tho maintaining the same goals as the last events:
a) to give the opportunity of putting old recovered or collecting equipment's on the air
b) promote an different experience as well calling, whenever possible, to this type of broadcast that it's not that used anymore, focusing on new ham operators
c) pay our respects to the old short wave amplitude modulation broadcasting in Portugal, nowadays extinct
The logbook isn't mandatory but it would be great if you send your log to the organization.
Those who can and are willing to give us that joy, please send them to cs5arla+dia.do.am@gmail.com
All participants that make a contact with CS5ARLA station, operated by Carlos Mourato (CT4RK), will receive a participation certificate.
 
João Costa CT1FBF
Vogal da Direcção da ARL"

PW Datacard, and AM

My Sept 2016 edition of Practical Wireless arrived today in the post. In it, was a data-card showing the HF beacons and on the reverse the latest HF band plans. Although I do not blame PW who just reproduced the band plans for our convenience I am again appalled that AM gets no mention apart from 5MHz.

BTW, am I alone in finding the 5MHz channels totally confusing?

For as long as I can remember just above 29MHz has been the home of 10m AM. Most users use old AM rigs and they sound superb. FM, with wider bandwidths, gets a mention, but not poor old AM. On 10m there is plenty of space for all modes. Why, oh why, does AM get treated like a nasty disease?? As 10m becomes more like a VHF band for much of the year there is a place for AM. Even in the best years, there is space. For much of the next 10 years 10m will be seen by many as a wasteland.

As readers will know, I like narrow-band digital modes like WSPR, JT65 and JT9-1. I also like AM which takes less space than FM, but is treated badly today.

AM and AM Modulation

Someone suggested I put this on my blog, so here goes with a brief introduction.

My favorite method is series modulation using a transistor as the "modulation" stage. The modulated RF stage is set so the stage voltage is close to half rail. With modulation the voltage swings to close to full rail (4 times the power). This stage is followed by a linear amplifier to bring the power up to that required.  I used this in my Fredbox and Sixbox rigs without a linear. If I ever do a simple Tenbox AM rig I'd add a linear RF stage.

See Fredbox .
See Sixbox .

Don't forget that if you click on a picture or schematic on my website a larger version is usually available.

AM rigs are simple to build. I am truly surprised that these have gone out of favour in recent times. Surplus PMR AM rigs can often be found for almost nothing.  Back in the 1950s and 1960s AM was the predominant mode on HF and VHF.

There is plenty of space for AM on 10m, 6m, 4m and 2m. A decent AM rig should only occupy 6kHz or less of bandwidth.

RSGB Band Plans

I have a love/hate relationship with my national society. In many ways they do a very good job. At the same time they seem to have a "thing" about AM.

Yet again, there is ZERO acknowledgement of 10m AM except a (begrudged?) comment that 29-29.1MHz is in the 6kHz wide all mode section. No explicit mention that this is where AM can be found. Indeed a year ago an RSGB official told me he had no idea about 10m AM!! AM has been just above 29MHz for as long as I can remember: got it RSGB?

The 2m AM centre of activity of 144.55MHz gets again a (begrudged?) footnote, whereas other modes get explicit mention.

No, the RSGB  hates AM. This is my conclusion. This is a stupid stance. There is plenty of space for AM on 10m and the VHF bands. AM gear is low cost too.

I love digital modes but AM has a place.

Portuguese AM Day

Southgate Amater Radio News reports that the Portuguese national society is promoting AM on HF and VHF bands on Sunday Nov 8th. The link shows where to look for others on AM. There is plenty of space for AM on 10m where the frequency recommended is 29.075MHz. On other HF bands there is less space for AM normally, although AM is popular on 160m and 80m.

See www.southgatearc.org/news/2015/november/portuguese_national_am_day.htm#.Vj3oyGtEd80

Fredbox - 2m transceiver

Many years ago I designed and made the 2m AM Fredbox 2m AM transceiver. At the time it was the smallest 2m transceiver I'd ever seen or used.  Several close copies were made.  It worked some impressive DX including several 60 mile handheld contacts and one over 100 miles to Brittany from the South Devon coast.  All these were with whip antennas on the rig and not beams. Most QSOs were with locals. The power was only 10mW AM. It was ideal for contacts around Cambridge where I lived at that time.

Some years ago, I rebuilt the transceiver and had some decent QSOs yet again.

As with the Sixbox, I would suggest the design is taken as a springboard for your own version. It is certainly ripe for further development.

See https://sites.google.com/site/g3xbmqrp3/vuhf/fredbox .

Sixbox QRP rig for 6m

Sixbox 6m AM rig - a simple design

Several years ago I designed and built a derivative of my 2m Fredbox but for 6m AM. The same basic design would easily translate to a 10m version.  Both bands could be useful for cross-town natters, especially when both 10m and 6m behave like VHF bands, which is in the quieter sun years and at night on 10m. The Sixbox is simple and certainly capable of further development.

Treat the design as a starting point. A 10m version of the RX has copied transatlantic USA AM stations well, but this is NOT a rig for DX use. It is better suited to cross-town/inter-village natters.

See https://sites.google.com/site/g3xbmqrp3/vuhf/sixbox .

Legal local AM broadcasting in the USA

On http://www.amateurradio.com/ there is a link today to the SSTRAN ANT3000 MW AM transmitter kit. This is legal in the USA for broadcasts around the home or with an FCC Part 15 compliant base loaded vertical for greater ranges.

Very many years ago my very first ever TX was an AM transmitter for MW using a crystal earpiece  a microphone. This was one of the circuits in my Heathkit Electronics Workshop which I got for Christmas 1961. It only got around the house. Another circuit was a MW receiver and I recall DXing with it that same holiday.

This kicked off my interest in radio and amateur radio that has remained with me ever since. I am not sure this TX was strictly legal in the UK. Years later I have reached the other side of the planet with less power!

AM on 28MHz and up

Much to my surprise, the RSGB actually published my recent letter to their "The Last Word" column in RadCom, together with a (pathetic) reply from their Spectrum Chair. At least my letter was printed.

The response did nothing to address my concerns. Although the RSGB claims not to be anti-AM, you'd find this hard to believe with a look at the most recent band plans. AM has its place on all bands from 28MHz upwards. I just wish the RSGB and other national organisations would simply acknowledge that this mode has its place. It may be a minority interest mode, but in my experience it is the simplest mode from a building viewpoint.

VHF AM and the RSGB

Recently, I wrote to the "Last Words" column in Radcom. I very much doubt my letter will be published. The main thrust of the letter is the (very negative) attitude of the RSGB to the AM mode.

My letter (as sent) is reproduced here. I'd be interested to hear your views.

At the present time G6ALB is experimenting with very low cost 70cms AM modules as the heart of a possible short range (3km) voice transceiver. We have had some success already.

"Thank you for publishing the latest Band Plans in the February 2015 RadCom. Although because of my poor voice (stroke) I tend to use digital techniques, I still enjoy the occasional outing using AM. Like many, I have worked transatlantic AMers on 29-29.1MHz with real QRP and simple antennas.

Yet again, one could be forgiven for thinking AM, especially 28MHz and up, is a dirty word at the RSGB.  AM gets no mention on 28MHz and 50MHz and gets a (begrudging?) comment as a footnote only in the 144MHz Band Plan when other modes get "centres of activity" mentions. For the avoidance of any doubt, AM is alive and well in the 29-29.1MHz sub-band. AM on the 144MHz (2m) band can be found on and around 144.550MHz. There has been AM on 29-29.1MHz for years and years and years! Yes, this is the all-mode section, but why not say this is the 28MHz (10m) AM sub-band? Also, why are 144MHz AM users asked to "consider adjacent channel activity"? AM should easily fit in 6kHz!!

Sorry, yet again, AM is being treated as a dirty and outdated mode. May I remind readers that ex-PMR AM rigs ripe for use on VHF can be picked up for virtually nothing and that there is room for AM on all bands from 28MHz upwards. AM has its enthusiasts on other bands too, but yet again the RSGB seems hell bent on killing off this mode. Why?

73s
Roger Lapthorn G3XBM"

Digital repeaters

As I have mentioned before, I am a rare repeater user, partly because my voice is so poor currently and I don't find operating through repeaters that satisfying. Each to his/her own and if this helps you enjoy the hobby go ahead!

At the moment it is far from clear which form of digital repeater will win: there are several competing digital standards and all have their advocates. For now I shall sit back and wait. A bit like the video standards war in the 1980s, the best system may not win. Let's see. Up to now I have only used FM repeaters.

When I do use voice I use SSB, FM and AM. All modes have their places on our bands. SSB is good on the crowded HF bands, FM useful for nets on VHF, and AM rigs simple to build and there is space on all bands from 10m upwards for all analogue voice modes. A well designed AM rig should occupy less than 6kHz, far less most FM transmissions on our bands. 10m AM, just above 29MHz, is a great mode. I have worked quite a few stateside stations on 10m AM. And I have only used QRP AM.  Let no-one tell you AM is a dead mode!

70cm AM test with low cost modules

This evening I listened for G6ALB on 70cm. Andrew is 3km from me. We both have V2000 vertical co-linear antennas. He was using a very low cost SAW locked TX module, which produced as much FM as AM when modulated with an electret mic. The biggest problem was Andrew's low power and low level of modulation.  He needs more mic gain, more TX power (10dB more?) and some pre-emphasis on the audio. TX power today was very low milliwatts. From 433.925MHz up to at least 433.990MHz a lot of squeaks and whistles could be heard, presumably from more local ISM devices. I was receiving G6ALB using an FT817 at about RS41 on FM and weaker on AM. This very initial test produced results that were expected. To use a super-regen module on RX (the intention is a very simple AM voice transceiver) will require G6ALB to be considerably stronger than on these first tests.

Fredbox derivatives

Since my original Fredbox 2m AM design, a number of derivatives have been designed and built around the world. This is what our hobby is all about. Although the original Fredbox worked well and its results surprised me, it was always ripe for further developments, which is healthy. My Sixbox was a 6m version and at some point, when fitter, I'd like to make a simple 10m AM version for local natters.

See https://sites.google.com/site/g3xbmqrp3/vuhf/fredbox for the original Fredbox. Click on the schematic to enlarge. Other derivative ideas are on my website www.g3xbm.co.uk.   By the way, it got its name from Fred G8BWI who was a disabled local in the Cambridge area back in the 1970s. Fred was a regular contact and he could talk for hours and hours and hours and hours zzzzzzzzzzz. RIP Fred.

433MHz AM modules - G6ALB progress

G6ALB has reported some progress in his tests using 433MHz AM modules, although it looks like an external linear audio transistor stage may be necessary to get best AM (voice) sensitivity from the super-regen detector. On TX he is getting 10dBm from the TX module at 5V (more with higher supply voltages) although the mod seems to be a mix of AM and FM. Our first "DX" test will probably be from G6ALB to me using the voice modulated TX module. I'll receive Andrew on my FT817ND initially. When the super-regen RX is sensitive with speech I'll try to receive him with that. Super-regens should be sensitive on AM, but are usually poor NBFM detectors.  We are about 3km apart and both have V2000 verticals externally mounted. At 70cms these have gain.

Andrew has ordered 5  pairs of 433MHz AM modules (TX as well as RX) for not much more than £2 total from China. This makes each TX/RX pair very inexpensive. It amazes me that they can make these so inexpensively. I hope G6ALB succeeds in this venture. It would be good to be able to communicate with him using a really low cost 70cm transceiver. I expect Andrew will write up his results in an article for a UK magazine later.

It seems entirely possible that a complete 70cm AM transceiver can be made for just a few pounds based on these low cost 433MHz AM modules.

29MHz AM

The 29-29.1MHz sub-band continues to be the best place to find HF AM. This is the HF band with most space, although there are AM enthusiasts on 160m and other bands. When these bands are not busy AM has its place.

When 10m goes "off the boil" 29MHz AM will be ideal for local nets with very simple gear. Ranges are fine for local nets. Also there is a ready supply of ex-CB kit around, although making a wholly homebrew QRP rig for 29MHz AM is a nice group project. There are some ideas for starters on my main website.

As an example of how effective 10m AM can be see http://www.macnaughtonart.com/10metlog-ss24.htm .