70MHz antenna as 145MHz antenna

I've been thinking of ways how I can hit two birds with one stone with having 4 meters and Two meters on one antenna. When I do the math a 70MHz antenna should resonate at 140MHz which is pretty close. As for the 70CM band, I get a lot of interference on 433MHz so 70CM simplex is generally unusable with that pulsing interference which seems to be all around the London area & in regards to repeaters on 70CM I can use a small antenna indoors for that so that just leaves 4 & 2 meters. 

OK 4 meters is dead pretty much but I want to see if I can encourage some activity on the 4 meter band as well as having 2 meters. I have limited space outside so I can only put up one antenna at a time, having an antenna that can do both 4 meters and 2 meters with an antenna switch would be ideal. That way I can switch between the two bands with ease. I will put up my 4 meter Viper antenna next weekend and try a two meter radio on it to see how the SWR is and if it works at all for 2 meters... if not I will need to think of a dual band 4 & 2 meter antenna design.

Any ideas on this?

Yes, the 3rd harmonic of 4m is a bit out to use 'as is' for 2m.

Will throw a suggestion out there to you to research if you haven't heard about previously... add a 2m tuned parasitic element for 2m to your 4m antenna. A half wave length of wire, rod, pipe or whatever tuned for 2m, simply mounted very close to your 4m antenna should bring down the SWR on 2m and make it more useable. Don't need feeder connected to it and so no switch box needed. Doing so improved my 2m j-pole performance on 70cm. It somewhat worked for me too when I added a 6m parasitic element to my 10m dipole (somewhat.. SWR was 2.0:1 at best on 6m, but useable).

I commend you for trying to encourage activity on a band that is somewhat quiet. For anyone trying to do this, be it on a quiet band or a little used operating mode, it can be a lonely time but rewarding when you do make a contact. Good luck.

Thanks for the info.

I'm going to do some experiments to start with... I'm going to make two flower pot style antennas, the first one will just be a simple half wave, effectively an end fed dipole and then the second one will be the experimental build where I use two half wave sections instead of two quarter wave sections and try that on two meters first. Theoretically this should work on two meters but because I'm using halfwave sections at 145 MHz as opposed to quarter wave sections it might just work for 70 MHz as well but I'll have to see what kinda SWR I get at 70MHz.

I am in need of another two meter antenna because the SWR on my main two meter antenna has gone very high and its been raining so I expect water has got onto something, so that has to come down tomorrow but I will need to replace it with something so I'm going to make a full wave two meter antenna which shouldn't take too long to do.

Also a full wave two meter antenna isn't too far off from a half wave at 70MHz, it does get a bit complicated so I don't know if I'll be able to do a 4M/2M dual band antenna. There is always the option of using an ATU to tune it for 4 meters but at that point its probably not worth doing because a halfwave at 70MHz is lousy as it is so anything worse than that is going to be pants. I'll just have to go with the 2 meter fullwave antenna which should also be good for 70cm. Only a 2dB gain according to Google at 145MHz I would have expected a bit more than that.

Actually scrap that idea, a double 5/8 wave would be the way to go if only I can find a diagram that is simple.

I have a lot of spare RG58 coax not being used so I will make a full wave 2 meter end fed wire dipole this afternoon with 9 turn coil at the end, I will use that for now, it be better than a half-wave.

Both antennas made and tested but performance was garbage on both. I will just have to use the crappy center fed dipole for 2 meters and search for a commercial 5/8 wave low angle for two meters. Not very much about at all.