20m dipole ATU tuning vs EFHW 4010

[rjvalenta]

new to HF here, always on FT8... i have been using a 4010 EFHW for a few months with an MFJ939i tuner to my IC7100. I was always able to tune the EFHW from 40-10 as expected but could also tune it to 60m and 80m and even 6m. but i had to position it in a way that was causing RFI into my home, so in an effort to learn and try something new i went and got a DXEngineering 1:1 balun and some 12ga stranded and cut a very resonant 20m dipole, starting at 16.7ft on each side and adjusting it.

eventually getting it to my roof, it works very well... the ATU will tune it from 10m, 12m, 15m, 17m, 20m (of course), 30m, 60m, 80m... but, it won't tune 6m or 40m?

it will tune 10m, 20m, 80m but not 40m...

this seems odd to me, shouldn't twice 20m or half 80m be something the ATU can handle? especially if it can handle 30m and 60m? and why can't it tune 6m?

and so now i wonder... if i had cut it for 40m would i be able to tune around as well as i could with the EFHW?

 

[prcguy]

A dipole is low impedance on its resonant frequency and odd multiples like 3X, 5X, etc, so a 14MHz will work ok on 42MHz, 70MHz, etc, with low impedance coax. At 2X, 4X, etc, its design frequency it will well over 2,000 ohms impedance and a horrible match to low impedance coax, something on the order of 40:1 or greater.

If you feed the same dipole with high impedance ladder line like 450 ohm or 600 ohm to the shack you can tune it with various kinds of tuners in the shack. At best it might do 40m on the low side but it should tune just about any frequency above that.