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 .

Poppet 160m AM transceiver

This little top band AM transmitter and a companion receiver were first published in the GQRP club SPRAT magazine. This TX version was built by M0DAD. Where the noise floor allows, 160m AM is quite popular for local nets.  There is something nice about “rolling your own” builds and getting satisfying results without spending a fortune. For daytime local use 160m AM is a great mode and rigs are simple. I am still surprised that more is not made of AM on 10m at night for local nets here in the UK.

See http://www.delboyonline.co.uk/m0dad/construcion/poppet_top_band_am_transmitter.htm.

70cms AM

As you may recall, I am quite a fan of super-regen receivers. Nothing as simple is able to match their AM sensitivity, but they tend to have poor selectivity.

With the availability of very low cost 433MHz modules (TX and super-regen RX) designed for AM data, these modules could be ripe for conversion to ultra-simple 70cm AM voice transceivers. One local friend, Andrew G6ALB, is currently carrying out experiments to see if this is feasible. I hope to work him on 70cm AM using such a rig in the months ahead. Sadly, I am still too clumsy to do any building work – very frustrating.

I am wondering if anyone else has tried these modules in such a circuit? With a simple MMIC PA (50 ohms in and out) the power output could be lifted to around 25mW carrier or more. With V2000 verticals this should be good for local ranges. Even at the low milliwatt level from these QRPp modules several km range should be possible with co-linear antennas.

UPDATE 2310z:   I wonder what sort of power the TX modules typically produce with linear (speech) modulation rather than 5V square wave data modulation? Presumably much less than their rated power, so an add-on PA may be essential?

AM and UK Band Plans

Because of my poor voice (as a result of my cerebellum brain bleed) I tend to use digital techniques, like WSPR and JT65, but I still enjoy AM.  Like many, I’ve worked transatlantic AMers on 29-29.1MHz with real QRP and simple antennas using AM. It makes a great change from SSB.

One could be forgiven for thinking AM is a dirty word at the RSGB.  In UK Band Plans published in the February 2015 edition of RadCom, 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. AM is alive and well in the 29-29.1MHz sub-band. AM on the 144MHz (2m) band here in the UK can be found on and around 144.550MHz. There has been AM on 29-29.1MHz for years and years and years – in fact almost as long as I’ve been active on the air. Yes, this is in 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!!

Yet again, AM is being treated as a dirty and outdated mode. Here in the UK, ex-PMR AM rigs ripe for use on VHF can be picked up for virtually nothing and there is certainly room for AM on all bands from 28MHz upwards. AM has its enthusiasts on other bands too, but yet again the RSGB seems keen to kill off this mode. Why I wonder?

Allegedly, a RadCom article on digital TV in the 146-147MHz band was pulled last month at the last minute because “someone at the RSGB” thought it would not fit in the new band! Sometimes one wonders. Maybe the day when I only get SPRAT is closer than I thought? Thankfully, there are many good articles in RadCom.

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