Band pass filter ordered. Look forward to trying it. Thanks Alan and Rick @M5RJC.
@2E0IYM - I’ll post an update on here just to keep you in the loop.
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In all my activations in the UK, I have never had one fail on VHF/UHF due to transmitter interference.
Only a handful of summits are impacted by (limited) interference, and there are ways to limit the impact hence I never found the need to purchase a bandpass filter.
Instead, investing in a decent handheld and antenna will reap greater rewards.
73, Robert
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Wendover is my local, at weekends you should be ok, on Saturday mornings their is a radio club on the local (Tring) repeater, you can call on that and ask them to go simplex, I’ve been up there many times during weekdays and got nothing, don’t overly worry about power, a handheld with an aftermarket antenna is all you need if anyone else is listening. My Signal Stick outperforms my roll up J pole on a mast so now I just keep it simple
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The inevitable wallet-depth dilemma. 
The filter will probably work with the next HT, too. It’ll get you out of trouble on those summits so affected, so worth getting if you’re going to do a lot of 2 metre activations. Also worth making something more effective than the standard rubber duck.
I made one of those for DF purposes a while back, and have taken it to a summit or two, but not had much luck with it. This is probably at least partly because I’m rubbish at HTs…
( Of course there are other bands if you want other ways of rescuing activations where 2 isn’t working… 
)
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I also carry a USB cable, audio interface and simple 3.5mm audio leads to hook up my KX2 to my phone (Google Pixel 6) for FT8, if I get desperate… …thankfully it is had never been required! The software called FT8CN is pretty easy to use.
Dave.
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Why bother carrying it, then? My philosophy is to minimize carrying redundant (radio) equipment [notwithstanding the spare pencil]. I refined my ‘pack list’ over years to minimize weight, setup/pull-down times and faffing about [BTW: I’m not talking about safely items like bothy bags and extra winter clothing]
If I were travelling a long way to the activation (especially an overseas one) I might have a bit more redundancy and definitely do a dummy-run set up/on-air test at home to ensure I’d forgotten nothing and everything works.
As I often want to do 2m FM (for local/regional chasers) and my KX2 doesn’t have VHF, I often take my 2m FM HT as well - that’s a bit of unintended redundancy. I know a couple of summits which are 2m FM dead zones so occasionally that pays off.
But I’m phlegmatic about the very occasional ‘failed’ activation - I (and my dogs) will have enjoyed the walking and it’s an excuse to revisit the summit another day.
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The leads and interface probably come to around 100g, so the weight vs. benefit isn’t bad. I live in South East London, so I am less phlegmatic than you are about failed activations, as you yourself note about having more redundancy on longer trips. There are a handful of 1 pointers within 90 minutes of where I am, the nearest 2 pointer is Coity Mountain GW/SW-012 in South Wales, and is 200 miles away! Any activations I do tends to be on weekends away or full on holidays, so I try to make sure the ones I do count 
As I said in an earlier post, I tend to pack for the summit attempted. As it happens, I am currently weighing all my kit and making a spreadsheet to come up with a couple of different ‘packs’ for different type summits, to help minimise weight and over-compensating with kit. I have both an IC-705 and a KX2 and the grass always seems to be greener on the other side! I like all the bells and whistles on the 705 with built-in 2m and HF, but the KX2 is simply such a good QRP rig with an in-built tuner that I only have to carry the single end-fed long wire to do 10-60m. I’m hoping weighing the kit will provide a decisive metric to pick one over the other!
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Getting back to the original question.
You have identified summits which have been successfully activated on 2m. I did the same piece of research on sotl.as last week.
2m the predominant band used on GW/NW-010
I travelled with just my Quansheng (£5), Slim-G (£20) and 4m mast (£20). Total cost of station £45 or £2.14 per QSO.
21 QSO’s logged, with 150km to a couple of stations
I could have probably used the Quansheng with a Chinese telescopic whip (£12), making it a £17 station, so 80p per QSO. However, the mast and slim-G combo allows me to sit, find shelter and easily log.
Masts, yagis, amps, ssb, low loss coax, VHF DX etc is a different (and fun) game. Much fun and many contacts can be had with basic low power kit.
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Good idea. I’ve a spreadsheet with every item I would ever carry in the first column and their individual weights in the 2nd. column. Then I have seven headings (and columns) for the main activation types I do. I tick [check] each cell as appropriate to include any item and the spreadsheet automatically calculates the total weight for each activation type. I’ve found this invaluable to seeing where the bulk of the weight is coming from and to ‘tune’ the list for minimum weight.
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If you had a suitable 2 metre rig you might even try FT8 on 2 metres… ( It is a thing, right? )
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I think that was the unintended consequence of my flowerpot antenna. 
I’m getting excellent TX reports, but it also picks up a lot of QRM (depending on where I am). Today on G/SC-011, it wasn’t a problem … but on G/DC-001 it was ridiculous and the radio was almost deaf.
The band pass filter has arrived today, so I’ll give it a try at the next noisy summit. 
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I’ve had a look at that link (a lot) over the past week.
I intend to buy new - looking for something that will last a very long time. Many of the well-regarded models are no longer in production… which is a shame.
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That sounds like a splendid day out and an excellent return on investment 
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You will find plenty of excellent condition handies available secondhand with a bit of searching.
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In all my activations in the UK, I have never had one fail on VHF/UHF due to transmitter interference.
Have you travelled up to GM-ES land yet and tried Brimmond Hill ES-086 just outside Aberdeen and very close to MM0EFI at Westhill and even closer to…?:
I know that the VX-7R that I use is notorious for getting interference from masts (often likened to barn doors…) so I carry a Sotabeam filter all the time and consider that the volume and weight is sufficiently small to make it worthwhile not to have to research each new summit carefully - and then remember to pack it! My tactic nowadays on a hill like this is to try with a short stick aerial on 2m fm which is much less likely to attract problems from the mast and only then to erect the dipole and dig out the filter - which unfortunately only works for 2m (when is the 70cm one coming?? The intereference on 70cm is getting worse every year.) Thanks to the super support from the local Aberdeen amateurs I managed to get 3 contacts on both bands and then I stuck - until Fraser MM0EFI tried to come to the rescue. We eventually made a 2m contact (of sorts) but were completely defeated on 70cm (5 contacts are recorded so I’m the most successful 70cm activator so far!!)
I was almost as close to the mast on Brown Muir GM/CS-120 on farmland
overlooking the Moray Firth (when the haar permits!) but, with the filter and a couple of s2s!) got 5 on 2m and 4 on 70cm. It is a pleasant walk up the mast road and the mast is a rather impressive structure
especially from underneath! I am sure that linking up with it would increase the distance my 5w signal travelled but it would be even bulkier and heavier than the anvil I usually seem to carry!
73 Viki
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That’s because the radio is not designed to use with an external antenna, its RF stages have little filtering and overload very easily. The external filter helps a lot in reducing out of band interference. The best fix is to use a radio that does work with external antennas in the first place.
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(Off Topic - a bit) As I set up beside what appeared to be the air traffic ground station on Forsnaval (GM/SI-200) ) (National Air Traffic Control base station - unmanned) I did wonder just how good the commercial radio filtering is on receive. I was working on the basis that if it causes interference to me then could the same happen to them? On this occasions I was on HF with no obvious interference, and I did consciously move further from their station to reduce the impact. A (very) long time ago I was working a mobile on 2m who was the station engineer for BBC Radio Nottingham. He went QRT about 100m from the studios as he didn’t want to cause any breakthrough. So… anyone know if the commercial stuff up the masts is relatively hardened to RFI of do they have issues with us?
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No, they use proper radios
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This!
I suppose, though, there could be a small risk from poor quality amateur kit radiating signals where it shouldn’t. Even a proper radio could suffer from in-band interference.
An add-on passband filter as suggested would reduce this risk too.
“Studios” implies a lot of audio equipment. I remember that 25 Watts of AM on 4m to a mobile whip very easily broke through on sound systems from a surprising distance (tens of metres).
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Yes, agreed. A few of my friends experimented today - cheap mobile radio Vs cheap HT (on the same flowerpot antennas). The mobile rigs did better.
Tried the band pass filter today, up on G/SC-003. Whilst it attenuated my TX, it certainly made RX a lot clearer (despite the relatively low QRM today). I didn’t see RX changing much on the S meter whilst experimenting with my friends - they just sounded clearer, so making contacts was easier.
Pleased with the purchase, worth having it in my kit. It’ll certainly buy me a more time to search for a better HT. 
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