20m dipole with end extensions for 40m

[KC1LML]

Thought I would ask before trying this since it's supposed to rain the next 3 days. I can easily lower & raise my 50' high 40m horizontal dipole to the ground. I was thinking about cutting the wire at the point of difference between 20/40m and adding an insulator with stainless steel bolts/nuts. The idea would be to connect the bolts for 40m but be able to disconnect them for 20m. Would the extra disconnected wire at the ends affect 20m performance? I know I could do this with loading coils and might do that later but thought I would try this first.

 

[n5ims]

You might first try to make a 40/20 fan dipole. Basically it's two dipole antennas in one. The top wire will be a standard 40 meter dipole and below that (but attached together at the center) will be a 20 meter dipole. Some tuning will be required since one dipole will affect the other but once you have the tuning right no further adjustments should be required. You'll actually end up with a 40/20/15 meter dipole since the 40 meter dipole will also work on 15 (but SWR will be a bit high although generally it's close enough). Be sure to keep the two dipoles separate except at the feed point. You can either fan them out or make insulated spacers (6" or longer recommended).

Often having the two connected will shorten the lengths by a bit, but initially cut them for the full length and adjust from there (it's easy to shorten but hard to lengthen the wires). It's best to tune the higher frequency bands first and then the lower bands, but you'll end up adjusting both either way. It's best to not cut the wire until you have both bands tuned. Simply twist the extra wire back on the existing wire until everything is tuned properly. An antenna analyzer makes this process much easier.

Note, if you want to improve your 15 meter performance, do a search on how to best do this. Many folks recommend a "bow tie" wire attached to the standard 40 meter dipole about 11 feet from the center (on both sides). This should also be tuned so don't solder it in place until you have the tuning correct and check the other two bands to make sure that they are still properly tuned and adjust as necessary. Adding this while the wire is new and still shiny will make soldering it in place much easier.

Some folks just attach the two dipoles at the same point in the center while others make a longer connection point to help isolate the fan elements. I know folks that swear by both so just pick which ever works for you. There are many designs and instructions on the web so I won't give you specifics here. My 40/20 fan dipole has great SWR on both 40 and 20 and with my built-in autotuner I can work all bands from 40 through 6. If I had the space, I'd add an 80 meter fan and hopefully could cover all bands but 160. My version is probably HOA friendly since it's mounted on the side of my fence. If I mounted it higher I'd probably have better results but in under 2 years I've gotten WAS on several bands and my DXCC so it must be doing something right.

 

[prcguy]

You can also get a pair of 20m traps then cut your existing antenna to the recommended size then tune for 20m, then add wire back after the traps to resonate on 40m again. That will make a 20/40m dipole that will be a little shorter than what you have now and work both bands without messing with anything. The 20/40m fan dipole is also ok but both have slight drawbacks like the fan needing to have the wires spaced apart at the ends and you have some degradation of the 40m element adding capacitance to everything when you are on 20m, etc. The trap dipole has a very slight amount of loss on the lower band due to the trap in series but it makes for a clean installation.

 

[WA8ZTZ]

Hy Power Antenna offers a two band 20 and 40 meter dipole antenna or you can homebuild it as prcguy mentioned.
Either way you don't have to be bothered raising and lowering the antenna every time you change bands
(like when its raining the next 3 days).

btw, don't forget lightning protection

 

[AK9R]

Linked dipoles are somewhat popular with the portable, SOTA, and POTA operators because they have excellent performance on the bands they are designed for with a minimum of wire to carry around. This page on the SOTAbeams website talks about them. At the bottom of the page is a link to a calculator that will help you design a linked dipole.

 

[prcguy]

Another thing you can do is replace the dipole with a 64ft long 40m OCFD which will cover 40, 20, 15 and 10m without a tuner. A 40m 64ft long EFHW will do the same.