Currently toying with an idea and was wondering if you all think it would work? So my idea is to take my current 20/40mm linked dipole and replace the current links with dual coil latching relays, small three wire cable ran along the 20m portion back to the center then back to the shack. Energize one set of coils for 40, energize second set to deconnect 40m section to run 20m. Dual coil latching replays so I don't have to keep the coils energized and possibly cause interference.

All the extra wiring for the relays would toss a big monkey wrench in the whole shebang. Somebody, a few years back, did propose a pneumatic system for controlling switches mounted out on the antenna. I "think" I saw it in QST.
73,
Rob

You don't really need to do all that.  Put up a 40 meter dipole and a 20 meter dipole.  Feed them from the same point in the middle (quarter wavelength per side on each).  The 20 meter wire won't be resonant on 40 meters and the 40 meter wire won't be resonant on 20 meters so it should work fine.  I used to run 40 meter and 80 meter inverted vee's fed from a common center point and they worked great.  The 40 meter wire will also be resonant on 15 meters since that is a third harmonic.

That's crazy talk, just make a fan dipole or use a set of 20m traps. The loss of the Unadilla traps from the market is notable.

Yup fully aware of all that, i just think it would be cool, and I just like screwing around with things. Probably my favorite thing about this hobby is making antennas.  I mean think about we are literally throwing wire up into a tree and feeding some voltage to it and talking 1000s of miles.

Be kinda cool though wouldn't it, flip a switch and you are running a full sized dipole with out the interference that you sometimes get from fan dipole. Plus it would be a little more stealthy than a fan dipole.

What "interference" would that be?

I've not made a fan dipole yet( it's on the list) but from what I've read since you are feeding the 20 and 40 at the same time then can cause distortions in your signal. Again no personal experience with them just reading about them and watching a few videos. If I'm wrong please feel free to correct me I enjoy learning about antennas and plan to make one.

That's kind of 5 out of 5 facepalms.

the wiring for the relays will detune the antenna for 20 and 40, and probably put RF in your shack.

a 20/40 fan diploe will work fine.

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Thanks for the replies and helpful comments may your day be filled with such helpful people.

From the ARRL Antenna Book. "The resonant length of any one dipole in the presence of the others is not the same as for a dipole by itself due to interaction and attempts to optimize all four lengths can be frustrating procedure. The problem is compounded because the optimum tuning changes in a different antenna environment, so what works for one amateur may not work for another."

Again I have no problem with fan dipoles, I simple had a idea and wondered if you all would think it would work, the consensus seems to be no due to the coils. I have since found out that MacTenna made one.

Not sure how well it works.  eHam has no reviews on it, and he's retiring.  No longer taking orders and website with patent is available for sale.  

We didn't say it wouldn't work just that it was a lot of effort for not much improvement.  I don't doubt that you could eventually make it work but I think it would be a frustrating project.  Now if you added a couple of RF-sensing switches that would automatically close the appropriate relays based on the signal transmitted you'd have something truly unique ;)

Or, to take your idea one step further, the quarter wave length on 80 meters is 8.35' different at 4.000 mHz and 3.5 mHz (band edges).  Twice that on 160 meters.  I had to add some stubs to mine if I wanted to work down in the low CW portion since it was cut for the phone portion of the band and the SWR curve is quite sharp.  It would have been nice to be able to close a couple of relays and have that done for me.

For what it's worth I didn't install mine as a fan dipole, the two dipoles were at 90 degrees to each other.  As I recall (and it's been over 40 years) I didn't have to compensate for having two in proximity to each other, both of them were cut to to resonant length.

Small change in resonant length isn't distorting the signal in any way.

Any change in the environment around an antenna can affect the resonant length. Most HF antennas end up being fine-tuned to their environment.

It is well known that the elements of a fan dipole are not the same length as a standalone single-band dipole. Higher band elements will need to be a little longer. This also varies with the angle between different elements

You'll get a better match in the SSB segment of 40 if you do this.