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I have an aluminum sided house, and when I'm using my HT I typically need to be near a window for it to work well.

Would I gain much if I put an antenna on the roof, an antenna in the middle of my house, and connect the two using coaxial cable? I assume it wouldn't be better than sitting myself with my HT on the roof, but will it be much better than the HT would be alone in the middle of the house?

I'd use antennas with matching impedance, but would probably use a very nice omni on the roof, and a smaller omnidirectional in the house. Does the type and length of coax matter very much, or can I use one I'd typically use for an antenna connected to the HT?

What else can I do to make this setup succeed?

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You could do that, but it wouldn't be terribly effective. The reason is simple: most of the power transmitted by your HT isn't directed at whatever antenna you set up.

You could probably do better by cutting the right sized slot in your siding. By forcing the RF currents in the siding to go around the slot, you set up an electric field which radiates as well as a dipole. This is called a slot antenna. Still though, losses will be very high, and probably not worth the effort.

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Assuming the HT is VHF/UHF, yes, the type of coax is starting to matter. For example RG-8X, a common HF grade coax, loses about 8dB/100'(33m) at 70cm. You are going to want something like LMR-400 (2.5dB/100' (33m)) and keep the run as short as practical.

After all, if you are losing 3dB (or more) in the coax and connectors (remember there is insertion loss for each connector), then you are at half power already. So, given the usual HT, that's down to 2.5W. Still plenty of juice to hit a local repeater, but you are using up the margin you might need to hit repeaters farther away or other folks on simplex.

What I have done is set a mag-mount with a spare 2m 5/8λ whip outside the window on a steel post and run the coax in through the window and use a connector to go from SO-239 to SMA-Female, I think. The coax was only 25', I think the total loss was less than 2dB.

You wouldn't have the freedom to wander around the house, but that wasn't one of my requirements because if I left the shack, all you'd hear would be the children screaming or something. After all, there is a reason shacks have doors :)

As for the coupling of the two antennas, I imagine it might work, but I think your loses would be so high as to not be practical if you need to get out with any power. Hard to saw canonically without more information and even then, it would probably come down to experimentation.

Reference for coax loses: http://www.therfc.com/attenrat.htm

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  • $\begingroup$ Insertion loss for connectors is an urban myth. The most commonly stated loss is .1 percent, which is 10 watts at 1000 watts out. The connector would get so hot you couldn't touch it. $\endgroup$
    – Larry
    Commented Apr 27, 2014 at 18:08
  • $\begingroup$ @Larry It's fairly commonly known that i.e. UHF/SO-239/PL-259 connectors do not perform well above 300MHz. This test shows about a 0.5dB to 1.25dB insertion loss at 70cm (commonly used by HTs as mentioned in the parent post): hamradio.me/connectors/uhf-connector-test-results.html - if my math is right, that is up to a 25% loss in transmit power from the connector alone in the 70cm band. $\endgroup$
    – Paul
    Commented Nov 15, 2019 at 22:00

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