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I've built a shortened 1/2 wave dipole for 80 meters to fit my lot in town. 90 feet total length. Using the calculator at K7MEM.com I opted for a 36 ft. and 9 ft. section (45 ft. on each dide of the dipole) with a 46 uH induction coil inserted between the sections on both sides of the dipole...

After much research (where I found counteless tips on the construction) I can find no guidance on the following:

When tuning the dipole to acheive minimum SWR on my target frequency (3.900 MHz) where do I shorten or lengthen? After the coil? Before the coil (between the feedpoint and coil)? Or, do I divide the required trimming equally between the two segments? - Or, perhaps divide the required trimming proportionally between the 2 segments (36 ft section is 80% of the 45 ft side of the dipole, so 80% of the trimmimg is done before the coil and 20% after the coil)?

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  • $\begingroup$ My gut feeling is that you could trim on either side of the coil, but of course trimming the outer conductor will likely be easier. Keep in mind you can also tune by adding or removing turns from the coils, and this might be better if you need to shift the center frequency quite a bit. This is the way I tuned a vertical for 20m to operate on 40m by using a loading coal. I was not confident in all of the calculations, so I added a few extra turns to the coil and then removed them one-by-one until I got the desired center frequency. $\endgroup$
    – gschro
    Commented May 23 at 4:37

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When tuning the dipole to acheive minimum SWR on my target frequency (3.900 MHz) where do I shorten or lengthen? After the coil? Before the coil (between the feedpoint and coil)?

The easiest would be adjusting the length of the two shorter wires after the coils. The most impact in moving the resonance around is by keeping all wire lengths as is and adjusting the inductance of the loading coils. For minimum SWR the antenna has to be at a certain height. Antenna height has a minor influence on antenna resonance.

I put the data you provided into EZNEC and it tells me the loading coils need to have 53.26uH and the optimal antenna height for minimum SWR is about 15m (49 feet). Dipole antenna made from 3mm (AWG 9) PVC insulated copper wire. Decreasing the wire diameter moves the resonance up which makes it necessarry to increase the inductance of the loading coils.

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