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Great Orme GW/NW-070 Quick Report

In reply to G3CWI:
That looks interesting , I presume that they are very loosely coupled.They look quite a bit smaller than I was contemplating. I am surprised that the sides of the box are not soldered as most of the sources I have read seem to think that any seams that run along the axis of the helix should be soldered, and that the two ends are not so critical.

In reply to G8JSM:

Found it:

“How-to-Make Better RFI Filters using Stubs”

Andy
MM0FMF

In reply to G8JSM:

What is needed is not only high Q resonators but also a resonator whose impedance varies more quickly with frequency. I have used several 3/4 wave length stubs on 6m to attenuate a TV transmitter just above band and give minimal loss in band. This is not an ideal solution for SOTA applications because of the added bulk and weight.

Doug
N7NGO

In reply to MM0FMF:
Thanks for looking. Hmm, where do I get 92 ohm coax, or do I use 2 lots of 50 ohm to get 100 ohm. It’s certainly a novel idea, I wonder if it is easier to make the line out of copper pipe, I wonder what happens if you increase the impedance further. Certainly food for thought.

In reply to G8JSM:

RG-62 (93Ohm) cable is commonly available. Just like you (and most other people) I don’t have any. It was used for Appletalk networking “back in the day”. I used to have miles of the stuff removed from an old place where I worked. I binned it all when I moved to GM 11 years ago. I knew one day it would be useful…

I think the fact the article starts with how to double and halve the impedance of coax suggests that using 50Ohm cable throughout is expected by people who don’t have a stock of less common coax to hand.

Andy
MM0FMF

Colin

My well stocked junkbox includes 92 Ohm coax but I’m not going down that path!

Here is a plot for the filter - finalised design:

Imgur

The return loss is 20dB (VSWR 1.23:1). It weighs 143g.

The skill (if there is any) is in the adjustment. The coupling is simply the hole between the boxes. This was painstaking adjusted with tin snips a few millimetres at a time. I think it’s probably very slightly over coupled at the moment. As with anything like this, all the parameters interact and it is very fiddly to get the right compromise.

There is more solder than might be apparent. There are blobs of solder in critical places and I have used copper tape to join the seams in some places.

I suspect it’s project doomed to failure (or potentially very poor results) unless you have a VNA to hand.

In reply to GW4CQZ:

In reply to 2E0YYY:
Mike,

For commercial radio sites like Great Orme (including paging and 3 FM
radio services) I use a homebrew band-pass filter centered on 145MHz -
although it has a little through loss on transmit it’s worth it to
hear the stations calling!!

Thanks Martyn, Just Googled band pass filters and I’m going to have a look at that as a club nite project.

73
Mike M6MMM

In reply to G8XTJ:

In reply to 2E0YYY:
Very interesting Mike. However you are unlikely to work anyone on
2ssb using a vertical antenna as the convention on ssb is to use
horizontal polarization and there is a huge loss if you use vertical

Cheers John. I’ve managed to work ssb verticaly from less noisy sumitts, however, as you quite rightly point out, it’s extemely difficult and not too much fun. To be perfectly honest, my sent signal and audio SSB reports, have on one or two occasions, been a tad on the generous side.

No such problem with FM though.

73 Mike
M6MMM

In reply to 2E0YYY:
That’ll trip everything out then :slight_smile:

In reply to G1STQ:

In reply to 2E0YYY:
That’ll trip everything out then :slight_smile:

Oh, so you’re back then? BTW, I forgot to bring you a stick of rock, back from Llandudno :wink:

73 Mike
M6MMM

In reply to 2E0YYY:
…and I forgot to bring you a beer back from Germany…

These pages may help the home constructor.

http://www.ifwtech.co.uk/g3sek/swxfiltr/swxfiltr.htm


http://tonnesoftware.com/helical.html

Alternatively I see that the DCI filters cost about £250 but look excellent. MFJ does one at about £78. http://www.mfjenterprises.com/pdffiles/MFJ-713.pdf

Cross Country Wireless also sell filters although the Raynet review seems to suggest they will offer only modest performance for what we want:

http://www.raynet-uk.net/technical/techfilt.htm

I would steer clear of any filter producer that does not provide a performance graph for their filter. Also remember that paging is at 136MHz as well as 153MHz.

Slightly off topic but who actually uses paging these days?

73

Richard
G3CWI

In reply to G3CWI:

Slightly off topic but who actually uses paging these days?

Apparently there is demand. Like you I was at a loss to think of who would want a pager with cell phones, SMS and mobile broadband etc. But one of the big advantages of pagers is that they are receive only (we’ll ignore 2way paging). That means you can take a pager into environments where you might not want a transmitter either for security or environmental reasons. Also the delay from sending a message to reception is often much less than cellular networks.

Still even with the above in mind, I’m still surprised there is enough demand to run national networks. However, if you’ve ever been near a paging TX site with a susceptible radio you’ll know the damn things never seem to run out messages to send. Blarrrrrrp, burble, booop, blarrrrrrrrp.

Andy
MM0FMF

In reply to G3CWI:

These pages may help the home constructor.

Thanks for the useful links Richard…

Slightly off topic but who actually uses paging these days?

Returned mine in 1987. First issued one in 1969 when I was working in the London Stock Exchange. It was a Multitone IIRC, quite heavy and used to ruin the top pockets of my suits. I think it gave out four short peeps when actived from the Stock Exchange trading floor …Oh and extremely limited range. Certainly did’nt work in most of the city bars :frowning:

73 Mike
M6MMM

In reply to MM0FMF:

In reply to G3CWI:

Apparently there is demand.

Indeed, I used to carry one around whenever I was on-call for her majesty’s weather radar. I probably even received a message or two from Billinge Hill. Mind I also used to carry the mobile phone around too and the first thing I’d do when I was paged was to pick up the phone :wink:

Iain, MM3WJZ