A simple antenna question

[KE6DF]

Sorry to ask such a dumb question:

If I have a 20 meter 1/2 wave dipole and want to work 10 meters,

is there any advantage to putting up a separate dipole for 10M,

or would I do just as well tuning the 20 meter dipole to matching with an antenna tuner?

I'm in one of those small lot CCR type subdivisions and my only option is to put up wire antennas under the eaves of the house that can't be seen.

[w5omr]

Depends on your feed line.  450-ohm or open wire and a tuner, no problem. 
Coax... well... it won't match well.

I think it will still radiate, but it wouldn't be as effective as a resonant antenna for that band.

[kd7qdu]

with a tuner, you may be able to work 15 and 17 as well as 10 and 20. 

Eric

[KE6DF]

The feed line is 50 ohm coax.

[Steve - K4HX]

The feedpoint impedance on 10 meters (28.5 MHz) will be about 4380 - j158 Ohms  (33.5' long dipole, 30 feet high over 'average' ground). This is a SWR of 88:1!

Assuming 50 feet of RG213 (RG8) coax, the loss in the coax will be 8.4 dB. That's 7.8 dB more than if the SWR was 1:1. Looking at it another way, if your transmitter puts out 100 watts, only about 15 watts will reach the antenna - not good.

These calculations do not include tuner loss which could be a few more dB.

Add 10 meter legs to the 20 meter dipole and skip the tuner.

[KE6DF]

OK I see how the coax mismatch would do me in,

Another option would be to run a random wire antenna -- say about 50 ft and bring the wire right in to the tuner. (No coax mismatch).

How would that do relative to the 10M dipole?

[Steve - K4HX]

It would be hard to tell without having geometry info.

You would be much more likely to have RFI issues with the random wire.

[W1AEX]

Add 10 meter legs to the 20 meter dipole and skip the tuner.

As a temporary measure that ended up being used for several months I did this with my 20 meter dipole last summer and it worked great. It was about a 30 minute project between adding the 10 meter legs and making a couple of cuts to get 10 meters resonant close to 29 MHz.

Rob W1AEX