I would hit a like but there is just too much to dispute mixing coax impedance for systems designed for a particular one.
If it were so then coax would not need anything more than who made it printed on it. Not going to get into a big lets drink a lot of beer and stand in front of fans and see who gets wetter first.
Two reasons you would do a mis match would be 1: It's what I have, and 2: I just don't care.
Antenna's have a characteristic value, receivers, xceivers have specific connection requirements. A 50 ohm antenna and 50 ohm rated radio with 75 ohm coax in between. Nah. Even if you have a SWR matching unit in between. It's going to do something it's not designed to do and there are a bunch of negatives in doing so. Sorry. It's so.
Commercial installations don't do it. The military certainly doesn't do it. Nor should you if you have the ability. Do it right.
I breezed through the white paper. So the Brits are still using an-a-lo-gway. Too much math. And coming from the folks who are infamous for Lucas Electrics. Pfft!
If there's anywhere in it where it states that a 75 ohm impedance antenna system and transmitter designed for 50 ohm impedance is okay to bullocks in 75 ohm Lucas coaxial. I missed it.
Kind of done here. A longwire, dipole, Yagi, most will need a matching device in between the feeder cable and them.
That Channel Master, Blonder Tongue TV antenna way up on the hill needs a 300 to 75 ohm balun if you use coax. Or 300 ohm twin lead.
If you don't then you get reflections (NTSC days), signal attenuation. Worse if you use a line amplifier.
Home Depot, Lowes because it's easy and fast. Or wait a few days and wait for FexEx. I choose for wait for the truck.
How about making the measurements yourself if you don't believe the hundreds, if not thousands, of engineers who have gone down this route before?
So, to unpick a few of these points:
If it were so then coax would not need anything more than who made it printed on it.
Specifications are important so no, understanding what you have and what you can and cannot do with it is similarly important. Note also that everything discussed here applies to *all* feeders, not just coaxial ones.
Two reasons you would do a mis match would be 1: It's what I have, and 2: I just don't care.
How about 3: This works fine, or 4: actually the impedance mismatch does something useful with the system I have. Or even 5: my receiver has an input impedance which does interesting things across its working range. Consider a dipole, folded or otherwise, centre-fed...what is the characteristic impedance at resonance if it's a half-wave? Why not use coax which matches the antenna impedance and then you only have one mismatch to deal with?
Commercial installations don't do it. The military certainly doesn't do it. Nor should you if you have the ability. Do it right.
Commercial installations *do* do it, as do "the military" since the designers of those systems (of which I am one) understand the what, why and how of what they are aiming to do.
I breezed through the white paper. So the Brits are still using an-a-lo-gway. Too much math. And coming from the folks who are infamous for Lucas Electrics. Pfft!
I'm glad you enjoyed the White Paper but I'm sorry that you feel there is "too much math" since this is an inherently maths-heavy discipline. That's not to say that using that all the time is necessary because it isn't but it is important to understand the basis on which estimates are made. I'm not at all sure what you mean by "an-a-lo-gway" but I feel certain that you can explain that for me.
But at that point you start talking about "Lucas Electrics" which makes no sense at all and continue to have a "pop" at "the Brits" which is simply unnecessary and perhaps belies your general attitudes towards others.
None of this detracts from the simple fact, calculated using widely discussed equations from first principles if you wish and then easily verified using modest test equipment, that the mismatch loss for a simple 50-75 ohm transition is small, typically around a quarter of a deciBel and is usually compensated for by the reduced loss in the 75 ohm feeder when compared to a 50 ohm equivalent. In fact, termination mismatch losses are quite easy to measure and compensate for, being discrete; what is much harder to characterise is the mismatch losses generated within a feeder due to the variation in physical dimensions, material homogeneity, etc.
Of course, in an ideal world one would design everything to be perfectly matched to eliminate all of these troublesome losses, making the system as a whole much more efficient and sensitive but most commercial, military and broadcast kit doesn't take this approach as it would all be too delicate and expensive; one makes it as good as can reasonably be achieved within the limits one has. The closest one gets to perfection is instrumentation and I'm sure everyone knows just how expensive and delicate that kit is!
In short, using 75 ohm feeder with a 50 ohm receiver results in very small mismatch losses but it should be remembered that most wideband receiver circuitry does not present a simple 50 ohm impedance at their input and often uses matching pads internal to their design. Basically, do the best you can within the constraints you have.
As with any hobby spend as much as you like as long as you can justify it to yourself... Feels good to have nice coax, I admit! But you're not a pro where you need to meet requirements... Find a scanner with a calibrated RSSI... good luck! Compare with your nearest ATIS perhaps, preferably UHF or ideally a steady FM signal or a dead carrier, maybe a 70 cm amateur beacon or repeater carrier. What's more important than the couple of dBs is to make sure you have not got a rotten antenna, rotten connectors or worst, water in the coax... Avoid "UHF" connectors at all cost!
