[TowerTalk] Relay Ratings Question

[trife at sirus.com]

Fellow TowerTalkers,

I am working on a method to remotely switch my Hytower between the 80 phone and cw sub-bands.  I need someone to give me a sanity check on the expected voltages present so that I make sure to get the appropriately sized relays.

Current configuration is the stock Hytower with the base loading coil.  The base loading coil is 3 inch diameter, 10 inches long and 4 turns per inch and looks to be 32 microhenries according to the B&W web site.

My plan is to place taps on the coil for each of: 40m and higher which will be very close to the top of the coil, 80m cw and 80m phone which will be an inch or so down from the top of the coil.  These leads will be connected to the switch box relays and remotely controlled from the shack.

My concern is the voltage across the relay terminals and making sure I size the relays large enough to handle the voltage and current expected at full legal limit power.  Since all of the relays would see voltage on their output contacts even when not connected to the center conductor of the coax because they are connected to the coil taps all the time and just switch in the coax when selected.

>From my cocktail napkin analsis, the inductive reactance of the whole length of coil, 2 x PI x f x L is about 700 ohms at 3.5MHz and climbs up to over 5800 ohms at 29MHz.  The voltage drop across this reactance at 1500W would be from right around 1000V at 3.5MHz to about 3400V at the top of 10m, V = SQRT (P x XL).   Thus, I would need coil contacts capable of about 1000V at 3.5MHZ to over 3400V at 29MHz.

For current, I would expect the majority of the current to be going into the antenna since its impedance would be much lower than that of the coil below the tap and ohms law would suggest around 5.5A at 1500W into 50 ohms.  So, with some head room, something around 10A would be very safe.  This leads me to vacuum relays appropriately rated.

Am I missing anything in this analysis?

Thanks/73,

Tim
KK9T