On 8/10/2015 8:30 PM, Wayne wrote:


"John S" wrote in message ...

On 8/9/2015 12:23 PM, Wayne wrote:
I was looking at a MFJ remote antenna tuner, and it is specified as
being able to match 12 to 1600 ohms.

What does that mean? At 12 ohms the SWR would be about 4.2:1 and at
1600 ohms a SWR of about 32:1.

If one started from 1600 ohms around a Smith chart, the circle would
intersect at the other side at about 0.64 ohms.

Likewise starting at 12 ohms and going halfway around the chart would
end up at 210 ohms, not 1600.

How does one use the specs if the antenna to be matched has a complex
impedance? Look at the internal tuner component ranges?

# What antenna are you thinking to match?

My yard is very difficult for getting antennas up. I currently use a
vertical on 20-10 that is mounted on top of a rather large metal patio
cover.
The cover does have AC voltage available for patio lights/fans/bird bath
pump and such. So I could power a remote tuner separate from the coax
if required.

The current antenna for 20-10 is a 14.5 foot vertical, fed with a short
run of good coax and tuned at the rig end of the coax.

IIRC, the swr is about 6:1 on 20 meters, dips around 18 MHz and rises to
about 20:1 on 10 meters.
For 20:1 the charts say I will lose not more than 3 db with the short
run of coax, which is acceptable.

In practice, the antenna is performing well enough for me to leave it
alone. ....but you know how hams are.....

I remember now. I even made an EZNEC model (including your metal patio)
and got results for all the ham bands. Using the specs for the
MFJ-998RT, it seems that it may match all but the 1.8MHz band IF the
specs can be relied upon.

I don't want to encourage you to spend a lot of money based on my input.
I just had fun doing the modelling.