So what you’re saying is I shouldn’t get a digital aerial right
Right
I’m not sure what you mean by ‘gain figures’.
The quick version. Every antenna has what is known as "gain". Put simply, it's how well it compares with a reference antenna design/type - and there is more than one reference type which is (ab)used by marketing types to make their products look better. Basically, the higher the number, the more signal you'll get. It's a logarithmic scale so 15dB is twice the power as 12dB.
But they don't "create" that extra signal. There's a tradeoff - typically you get the extra gain in one direction by not working as well (or at all) in other directions. So the higher the gain, the more accurately you need to point it. As an analogy, two different torches could use the same bulb (or these days, LEDs) - one could have a bright but narrow beam, while the other has a less bright but wider beam.
Manufacturers, or rather the marketing people, like to use the best figures possible. So for aerials they usually quote peak gain figures. If you look at the graphs I linked to, you'll see that the gain figure for an aerial depends on the frequency - which is why it's vital that you know what you are trying to pick up. Hemel Hempstead is a "group E" transmitter - you need either a wideband aerial, or a group E aerial. Generally, a wideband aerial works across the whole TV band, but it's a bit "jack of all trades, master of none" - while the Log40 (or Log36) is reckoned to be the best "all rounder" - it still won't beat a similarly sized Yagi that's designed for just the band you need. If by mistake you bought a Yagi 18 A (that's the band A version), you'd find it would pull in very little signal from your group E transmitter despite being a very large aerial.
Taking specifics, from the table you posted earlier, you need to receive channels 44 through to 59. Looking at
http://aerialsandtv.com/gaincurves.html#EgroupCurves you'll see that the Yagi18E gives better gain than the Log36 over this range. If by mistake you got a Yagi18A, you'd find that it gave very little signal at all - the gain chart is further up the page and you can see that it's response drops off the bottom of the graph at around channel 43 !
So TL;DR version - more gain = more signal.
But you have to check what the actual figure would be for the channels/frequencies you need. That's why it good to use a vendor that gives you the graphs - rather than one that hides this information, lies about it being a "digital" aerial, and further lies by including an amplifier to get the "good" gain figure.
Hemel is only 3 miles away and there is no terrain in the way, so I would assume this would be the best one.
Most likely. It's much lower power, but if you have clear line of sight then it's a good bet.
It's not an area I have much expertise in, but I suspect just the small DM Log aerial would work for you. A Yagi 18E certainly would though it's quite large. IMO it's better to have too much signal as you can reduce it very easily (fit an attenuator). If you have too little, it's hard to boost it without adding noise.
Crystal Palace is 22 miles away and the other side of London. I would have thought there would have been a massive amount of signal degradation with all the buildings and additional airwaves traffic caused by a big city?
Buildings yes, but other "airwaves" shouldn't be a problem. If it's 22 miles across London, there'll be a lot of tall buildings, that will have a significant impact.
How does the nanobeam wireless bridge work? Just attach it to the garage, point it at the house and connect it to the wifi as I would a laptop or smartphone? Would this then just give me strong wifi in the garage? Would I have to connect a device at the bridge end through an ethernet cable, or does it transmit wifi back out again? I'm thinking if it transmits wifi out, then I can just get a chromecast or something and pick up the signal. But as a chromecast doesn't have an ethernet port, it would need to pick up wifi.
There are lots of different options out there.
Ideally, you'd use two Nanobeam devices (using two, they use a proprietary wireless link to get better throughput). Each one has an ethernet port so it looks just like a long cable to the devices at each end. You can use just one, it will connect to your WiFi and give you an ethernet port which is very similar to plugging into the ports on the back of your router. it doesn't repeat the WiFi though.
You can also get WiFi repeaters. These connect to your WiFi, and 'rebroadcast' it - thus extending range. They generally use the same antenna(s) for both - so I don't think you can use a highly directional antenna to get to the house without compromising the local signal.
What you can do is use a client bridge (eg the Nanobeam or anything else with the same function) and connect a WiFi access point to it with an ethernet cable. That way you separate the functions and pick the right option for both parts.
