My 60′ (18M) wire vertical for 80M covers about 30 khz at the same SWR. This means I will be good to go in the CW portion of the band (center the “long line” for 3.515), and then cover where the nightly net is (center on the “short line”).
246 divided by the Frequency in MHz = Length in feet
246 / 3.515 = 70′ (5′ up and back down)
246 / 3.857 = 64′ (2′ up and back down)
The ladder line stubs plug in on the top right and left of the box, and the center feed through is for the vertical wire taped to the Spiderbeam 18M (60′) mast:
I can actually just hard wire the coax in shield directly to the radial (gnd). There is no need to bother going through the relay. I’ll keep the leads very short and close to the terminals – which can be bolts – the same that Balun Designs uses since the box is plastic.
With two pieces of ladder line and 3 relays, I can switch between 75 and 80M and be covered perfectly. I didn’t have this issue on 160M, because I was 95% percent of the time between 1.815 and 1.830 – so only 15 khz for almost all DXCC QSO’s.
The best thing about this is there are no coils or capacitors that get hit with very high voltage, and I think this little bit of linear loading (16%) means virtually no sacrifice in performance. The Elecraft gear – especially the KPA-500 will like this. 80M has always been a bit “finicky” with the KAT-500 and the KPA-500 . . .
The default (position 1 on my switch box) does nothing – its not even hooked up, and its the 75M CW band portion, and Position 2 is for 80M, and a voltage is applied. The two relays are wired “opposite” each other – one is NO and the other NC when voltage is applied. This means there is no chance that a path is open where both are switched in. I also do not add the length of one stub to the other.
I will build the relays in a small box and also use a current choke that is always in the circuit. I’ll tune using the Rig Experts AA-30, so this will be pretty easy – a couple hour project. One thing I have learned – it is FAR better to be a little long than too short. Now I really understand why shortened verticals on the low bands lose efficiency as severe as they do – and also understand how you can move the high voltage and current nodes around based on size.