"If that is the case, how come some people can transmit via their rain gutters and using a tuner? "
Any antenna, feedline, and tuner combination radiates a LITTLE bit of power at least.
If your Interceptor antenna is tuned for a good SWR on 10m, for example, and you feed it with coax and a good tuner on the 17m band, you end up with a TON of feedline loss and, therefore, a very inefficient antenna system.
However, the cool thing about ham radio is that you don't need to radiate much more than watt or so to happily make *some* contacts.
People prove this every day by running 1W to good, efficient antenna systems. I've worked Norway on the 160m band using approximately 1.25W effective radiated power.
On 17m you could easily work DXCC with 1W power output (maybe not right this moment in the sunspot cycle, but it can be done).
So you can go ahead and "load up" your severely mistuned antenna on 17m and successfully radiate 1W out of your 100W applied from your barefoot HF rig, the other 99W going up as heat in the coax due to mismatched line loss or as heat in the tuner due to the nasty load the antenna will present.
That *WILL* make contacts. You'll probably even get a few "you're a good 5 by 7" signal reports. That doesn't mean your antenna system is working worth a darn at effectively radiating the power you're applying to it.
Now, the fundamental problem really is usually feedline losses. If you actually put your matching network right at the feedpoint of the Interceptor, like if you had an autotuner or a remote controlled tuner, the Interceptor might work pretty well. If it's a 5/8ths wave on CB it's actually pretty close to a 1/4 wave on 17m and with the tuner *right at the base* with no intervening coax cable, and with the interceptor's special matching network taken out entirely, it could be an OK 17m through 10m antenna. It would help if you added radials for each band, or at least lengthened the ones that are there.
This is how the rain gutter folks tend to be most successful. They buy an autotuner, and attach it right to the downspout or something, and attach the other side to some hidden ground radials, and they do pretty well.
When I was in an apartment, I compared the two approaches directly: tuner in the shack with coax and tuner out on the balcony, remote controlled, basically at the antenna feedpoint. The difference on some bands was night and day, and probably to the level where I was radiating at most 1W or 2W of my 100W on those bands.
This is how I remote controlled my regular tuner:http://www.n3ox.net/projects/servo
There are good ways and bad ways to feed random length antennas on multiple bands, but if you don't like to do a lot of planning and calculations, putting an automatic matching network at the antenna is often the way to go. I do like to do a lot of planning and calculations, and I still tend to put the matching network at the antennna:http://www.n3ox.net/projects/lowbandverthttp://www.n3ox.net/projects/sixtyverthttp://www.n3ox.net/projects/2017moxon
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Anyway, if you want to try loading up the Interceptor through some coax, just be aware that making some ham radio contacts on it will be POSSIBLE even if the whole system is working VERY badly.
Probably the simplest way to get a really great antenna right now and to have the most fun using the Interceptor, if it is just a CB 5/8ths wave stick of aluminum is to turn it into a single band 17m groundplane by taking out it's matching network and adding a few 17m 1/4 wave radials.
15m and up are rarely open these days anyway.