[Elecraft] Re: may have it wrong

[wayne burdick]

Hi Mike,

>
> For frequency 7118,  I took a length of bare copper wire 33 feet long, 
> divided (that is cut it ) it in half, took one half to the center of 
> the BNC connector on the side of the radio and the other half directly 
> to the grounding rod attached to the house.   but from reading the 
> emails on the group list it now appears I need to have a insulated 
> coax and I should not be connected directly to the radio with the two 
> bare copper wires.....

You can certainly get away without any coax at QRP power levels, 
especially if you use an antenna that's close to a quarter wave long. 
It's OK to directly connect the wires to the KX1. But what you 
described won't work very well, because both wires need to be about 33 
feet long at this frequency. By cutting one 33-foot piece of wire in 
half, you've made a 20-meter antenna  :)

The formula used to calculate the total wire length for the antenna is:

    468 divided by the frequency in MHz

So:

    468 / 7.118 = 65.75 feet, or 65 feet 9"

This is a *half-wavelength* of wire. To connect the antenna to your 
rig, you would then feed it in the center, where the impedance is close 
to 50 ohms (this matches the impedance of the rig's antenna jack). This 
of course involves cutting the wire in half. But you may want to add an 
extra inch or two at each end for use with antenna insulators, etc.

The KX1 will be perfectly happy with this antenna. You can simply 
insert one wire into the center terminal of the rig and toss it into a 
tree (etc.), and connect the other wire to the rig's chassis and lay it 
on the ground. In fact this is what we designed the rig for: ad-hoc 
1/4-wave or random-wire antennas fed without coax. Your SWR may as high 
as 3:1 if you don't trim the wire to length using an SWR bridge, but 
the KX1 can handle high SWR all day, so give it a go.

If you want somewhat better performance, you can reconfigure the 
antenna as a dipole or inverted V, the higher off the ground the 
better. There are many examples in the ARRL Antenna Handbook, which I 
highly recommend. This, of course, will require coax, or balanced 
feedline and a balun (balanced-to-unbalanced converter; the KX1's 
antenna input is what we call "unbalanced," no reflection on the 
designer, yours truly  ;)

You might also consider using an antenna tuner. The optional KXAT1 will 
give you several advantages:

   - a good match for the transmitter, minimizing power loss
   - operation on multiple bands with just one wire antennna
   - power and SWR indication on the KX1's LED bargraph
   - the ability to peak up the antenna on receive manually outside of 
the ham bands
     (using the "G00-G31" selections in the KX1's ATU menu entry)

Our new T1 ATU will also do the job (less the manual tuning 
capability), and can serve as an ATU for the KX1 and all other QRP rigs 
up to 20 watts. But you can't match the convenience of the KXAT1.

73,
Wayne
N6KR


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