[Elecraft] More KAT500

[Jim Wiley]

Ed  -

According to Elecraft, the KAT500 can match 10:1 SWR at 500 watts, on 
any band 80 to 10 meters.  It can also match 3:1 SWR at 1000 watts on 
those same bands.   The 160 meter and 6 meter capabilities are a bit 
more limited, but it will still match a fairly wide range of loads.  By 
inference, 10:1 SWR at 500 watts between 3 and 30 MHz could result in 
peak voltages in the range of 2250 volts and peak currents of 42 amperes 
RF  (one or the other, not simultaneously). This is pretty impressive 
for a box that size.

Using those voltages as limits, that indicates that the KAT500 could 
handle power levels of about 250 watts at a 14:1 SWR.  Any more could 
result in damage.  I would suggest replacing the 300-ohm line on the 
antenna with 450-ohm ladder line, and inserting a 4:1 balun between the 
antenna and the KAT500.  I suspect that this will reduce the SWR 
excursions, and may get you "inside the envelope" for what the KAT500 
can handle.

A commonly used antenna is a doublet (dipole) fed with open wire, a 4:1 
balun, and a coupler.  Most installations use a length of RG-8 coaxial 
cable (typically 10 to 20 feet, but no longer than necessary) to "get 
through the wall" - or from the coupler to the balun immediately outside 
the building, then the open wire line connects to the balun at that 
point and continues to the antenna. 450-ohm ladder line is usually a 
better choice because it is designed for transmitting use, where 300-ohm 
line is usually "TV lead-in" and not particularly suited for use at 
higher power.  Some transmitter rated 300 ohm line exists, and if you 
have that type, it may work for you.  As with all open-wire type lines, 
keep the line at least 8" to 12" from metal objects if possible, more is 
better, and avoid abrupt right angle turns.

- Jim,  KL7CC





On 11/15/2012 2:41 AM, Goss, Ed wrote:
> Hi Jim,
>      Thanks for the info.  I am at work now and unable to post back to 
> the group, but I appreciate the explanation. For the low bands, I use 
> a 135 ft inv vee center fed with 300 ohm line, with a 1:1 balun near 
> the shack. When my KAT500 starts to tune itself, I am seeing high 
> SWRs, such as 14:1. After the auto tune cycle, it's 1:1.  Perhaps I 
> should be concerned that even with 100 watts, there would be 
> conditions that overstress KAT500 internal parts with this type of 
> antenna, but I want to be very careful about not damaging the tuner. 
> I'm not quite sure how a user would know these limits, or if the 
> KAT500 protects itself in any way. Perhaps the KAT500 is meant to work 
> with antennas that present a somewhat better match to start with, like 
> a typical 80 meter dipole which is cut for CW but the user wants to 
> use all the way up to 4.0 MHz...
> --Ed---
>