BATC Forum

Now that the 4m extension band has been made more widely available, I have been researching suitable equipment. Below are some ideas that I have found. No doubt members will have further ideas, or perhaps reasons why these won't work!

TRANSMITTER
The Portsdown will work at 71MHz. I have a DATV Express and the website specification says 100MHz upwards. However, the picture below indicates that mine is working well at 71MHz. This is 333ks/s with the Express at mid output level.



POWER AMPLIFIER
Anglia Live list a Mitsubishi black brick module type RA30H0608M-101. This gives 30W out for 50mW drive, maximum output 45W. The frequency range is 68-88MHz. Supply voltage 12.5V. It looks similar to the types we already use for 2m, 70 & 23cm and can be biassed for linear operation. GM3SEK has produced an excellent application note for these modules, including values for a 4m version with low pass output filter; http://www.g4ddk.com/AN4.pdf A PCB is available from G4DDK.

FILTERS
These Pye filters are often seen at rallies. This one is the rarer VHF Highband one for 2m, but I have discarded loads of the lowband 68-88MHz ones when looking for these.



RECEIVE UPCONVERTERS
The mk.II Minitioune will tune down to 2m. Spectrum Communications produce a range of receive up converters, 10, 6 or 4m up to 2m. They feature a MOSFET front end with some filtering, crystal controlled LO and mixer. They are available as kits or ready made. This one is for 6m, I used it for converting Band I TV carriers up to an SDR.




AERIALS
Sandpiper produce a 3 and 4 element beam for 4m. Boom length is 2.2m, element length 2m and it is gamma matched.
Moonraker produce a similar 5 element beam and a smaller 2 element HB9CV.

I am hoping to get a portable station working soon, probably on receive initially.

73 Shaun G8VPG.

Shaun,

Express will go down to about 65 MHz, the 100 MHz figure is the rated lower frequency from the manufacturer (ADI) not what it will actually do.

- Charles

I have been busy constructing my 4m portable equipment and everything is now complete. I have built the Spectrum Communications up converter and it has received signals across the bench. Some people think that having the LO at 75MHz, close to the signal frequency, might be a problem. Also, it has only one gain stage and might need some post-mixer gain at 146MHz. We shall try it and see. Lots of people are working on various ideas for this.
The Mitsubishi RA30H0608M amplifier has now been built and is driven from DATV Express.

Much to my surprise, it has enough gain to go to full output with the Express without the need for a further driver amplifier. The picture below shows the shoulders at 10W, -35dB (power measured with 100-250MHz Bird slug).

The low pass filter in the output stage is doing a good job, with only the second harmonic at -55dB visible up to 1.5GHz. This is at 20W output, shoulders -30dB. I pushed it to 25W, but it was getting a bit wide there! I think 10-15W is a reasonable level for this amplifier.

Today the aerial arrived. It is an Innovantenna 3 element OWL yagi and the company cut a special for 71MHz at the standard price and delivered it in less than two weeks. I have got an SWR minimum of 1.1 at 71.1MHz - that is close enough for me!

To complete everything, I have built a basic Portsdown system with the 7" screen, EZcap and RTL dongle for portable use controlling the Express.

Hopefully there will be some 4m activity on the next activity weekend, although I can go out beforehand in the Bristol/Bath area if anyone wants a sked. There are some more pictures on Twitter @g8vpg.
73 Shaun G8VPG.

Good stuff,

I think the largest challenge on receive will be avoiding interference from broadcast FM causing issues to reception.

As on 146MHz, -35 dB shoulders are a bit high as they fall outside the 70-71.5MHz range at 333ks, which can be seen from your photo if I am reading it correctly. I would suggest reducing the power or symbol rate so that signals above 71.5MHz are at least 60dB down. How much do you get out with -50dB shoulders?

Mike

Hi Shaun,

My setup is giving comparable results.
At 14 Watts the shoulders are 35dB down; It's the same PA block but I bought the complete kit from Minikits. It worked well at 50MHz when we did tests a couple of years back.

Initial testing at 71MHz was with the Portsdown driving the PA via a filter only. I have since used the "Portsdown Express" to drive the PA with a much tidier constellation.

I'm using a 3 element Sirio beam currently with 10 quid off at Moonraker.

All being well on Sunday the 15th July I will be on Dunstable Downs from 10am till about 13:00.

73 Steve

Well I seem to have a working station:

DATVexpress with Portsdown UI in to same brick gives about 12 watts with 40 dB shoulders.

On receive I have a MMIC pre-amp in to a bandpass filter which is the easy to build 4 pole 146MHz design https://wiki.batc.org.uk/File:146_filter.JPG with the coils squeezed together a bit and an additional 10pf across one of the tuning caps. Works well with 1.5 dB insertion loss, reasonable shape and - 6odB in band 2 which should give enough protection.


I have a sleeve dipole which seems to tune....

Unfortunately I'm not around much next week or the weekend but could do some tests on Monday / evening from Walbury or similar.

73
Noel

OK Noel,
Perhaps we could pencil in Monday evening, 9 July. I was thinking of somewhere quite local, Bath Racecourse, Lansdown, IO81TK, 228m asl. The take off is quite good to the East. How does it look from your end?
73 Shaun.

I have borrowed one of the Pye 68-88MHz lowband filters that looks exactly like the 2m one in my original post. This has lined up very well with 0.25dB insertion loss, 88MHz 33dB down and 108MHz 47 dB down. I will do the other measurements suggested soon, but it is a bit too hot to work in that room at present!
73 Shaun.

Shaun,

I have no experience of the LF bands but looking at the path plots it seems to go a long way even on a dipole!

Most of the sites I use, tend to be on ridges running east / west and so are good to the North / South but I'll sort something out - be fun to try and best DX seems to be about 4Km at the moment!

73
Noel

Using Low Band AM in the 70Mhz area, some years ago, mobile to mobile, in the welsh mountains and valleys it got almost everywhere

It was very popular with Vets across the country and covered fairly large service areas from a given base / repeater site.

Wish I had hung on to those PYE filters, what are we going to need those for

Wonder if still used in the hard to get, out of the way places now we have the mobile phone?

Peter