Fan Dipole Queries

[M7OJW]

Hi,

I'm planning to construct a fan dipole for the following bands 40/20/17/15/10m I realise that the 40m element will also probably work in 15 as 3/4 wave, but will lobes develop on 15m in this case? If I have a separate 15m element, will the self selection on the basis of impedance favour the 15m element when using the band rather than the 40m element? I wasn't going to have a separate 12m element, will the 10m element likely tune on 12m with the internal tuner of my ft991a or will it be outside of its range? If anyone can advise as to the above, I would be very grateful. How far apart should the elements be spaced? Is there anything else, possible problems etc that I should be aware of with this type of antenna?

Many thanks for any help that anyone can give.

73

Owen 2E0IHR

 

[KA4EPX]

I’m using the formulas from Antenna Calculator - West Mountain Radio

Antenna Calculator - West Mountain Radio

www.westmountainradio.com

I’m building a three element, fan dipole, with an 80, 40, and 20, resonant at 3.940, 7.150, and 14.200.
The 80 on top, 40 in the middle and the 20 on the bottom.
For spacers, I bought a 10’ section of 3/4” 200psi pvc pipe. Cut 8 pieces 9” long and 8 pieces 5.5” long. I split them vertically, used a heat gun to flatten them out. Marked them 1” from the top and 3/4” from the edge. The 9” piece has six holes. The 5.5” piece has four holes. The 9” piece will be placed at four foot spacing, and will end at the end of the 20 meter section. The 5.5 “ pieces are spaced at roughly four foot spacing til the end of the 40 meter section.

I’m using #10 stranded insulated THCC wire, as it’s what I had on hand. 12 gauge is what I used in other standard dipoles.

For holes in the flat pvc pipe, I used a 11/64” drill bit. I put a wire tie around the wire and back thru the flat pvc pipe halfway between the two holes the wire was run thru as a way of preventing movement of the wire in the flat pvc plastic.

I’m using a current balun in the middle fed by LMR240 coax, with a strain relief so the PL259 doesn’t have to carry the weight of the coax. It’s a simple loop of coax, using a wire tie, and a piece of 1/8” paracord holding the tension to the balun.

It’s still being built, but the friend who has this type antenna, says it will tune all bands up to 30mhz except 17 meters.

 

[KF5LJW]

Over the years, I have used a few fan dipoles. Performance depends on many factors, most importantly, height above dirt and horizontal clearance from other structures and trees. Height is your greatest challenge unless you intend to operate short- and medium-range. Elevating the dipole to ½ wave is easy to do on higher frequencies like 10 and 12 meters, becomes challenging at 20 meters, and 40 meters out of reach for most needing 66 feet of height for acceptable DX.

The 40-meter element will interact with the 15-meter element. Essentially, you have two elements in parallel and subject to parallel circuit laws. If the two elements' impedance = 75 Ohm in parallel with 75 Ohm = 37.5 Ohms or VSWR = 1.33. The pattern will depend on the height and nearby objects. One thing is for sure: if it is lower than 66 feet, rest assured that the 40-meter dipole will be a cloud burner and earthworm heater.

Your radio auto-tuner is limited to 3:1, and I doubt you can tune 12 meters on the 10-meter element. You will have to try, I have not run the numbers.

Having said all that, I learned a long time ago to run a doublet antenna fed with a ladder line and a good tuner. Determine the lowest frequency band, and you can tune it to any higher frequency.

Whatever you decide, install a maxi choke at the feed point.

Good Luck.