[Elecraft] A Good Antenna Length?

[Don Wilhelm]

Jim,

Having one antenna to cover 160 meters through 6 meters is asking a *lot*.
But I would suggest starting with a halfwave on the lowest frequency band.
If that is a half wave on 160, then the pattern will begin to break into 
multiple lobes beginning at 20 meters and up.  Whether those lobes will 
be in a favorable direction for you is a different question.

Of course, you would feed such an antenna with open wire line or ladder 
line because those are relatively low loss and losses will increase as 
the SWR increases.  The feedline will have a high SWR on some bands.

The length of the feedline makes a big difference in the ability of any 
given tuner to resolve the impedance at the shack end.  A few sessions 
with the TLW (Transmission Line for Windows) may be helpful in 
determining the best feedline length compromise that ends up with a 
usable impedance at the shack end for all bands considered.  Of course 
it will be necessary to know the antenna feedpoint for each band to know 
how to find the impedance transformation that will be present at the 
shack end.  Antenna modeling can answer that antenna feedpoint impedance 
question.

If the feedpoint impedance at the shack end is out of range of the tuner 
being used, some additional capacitance or inductance placed either 
across the feedline or in series with it may be necessary to bring the 
impedance into a range that the tuner can handle.

Thirdly, you need a good current mode choke (balun) to keep RF out of 
the shack and to provide a balanced to unbalanced transformation.  Its 
impedance must be at least 10 times (more is better) the highest line 
impedance seen at the place that current mode choke is placed.

All the above must be taken into consideration for any antenna.

Yes, there is much more to your question than just the lengths of the 
radiator.  Any answers that do not also include the type and length of 
the feedline may not be able to be duplicated given your particular 
physical situation.  Antennas and feedlines are just like that.  Of 
course an antenna whose feedpoint impedance is matched to the 
transmission line characteristic impedance can use any length of that 
feedline, but that is not the usual case for multiband antennas.

You will likely have better luck with 2 antennas - one for the low HF 
bands and another for the upper HF bands - those are normally easier to 
deal with.  The ideal is a resonant antenna for each band, or fan 
antennas covering multiple bands (I restrict those to 3 bands because 
the interaction makes tuning frustrating).

Good luck on finding that "magic" length - many have tried over the 
years and all are compromises.

73,
Don W3FPR

On 7/4/2014 5:21 PM, Jim GM wrote:
> What antenna lengths are you using on 160M with your KX3 or  KXPA100
> internal tuner? What is best for 160-6 meters? I like making my own
> antennas with wire.
>
> I usually have to give up one band or another cause it just would not tune
>   up on a certain band.
>
> I have tried to stay with in these guide lines.
>