The "Kinda Random Antenna" (was: Random wire lengths for antennas)

[wayne burdick]

I'd call an ad-hoc antenna that works on multiple bands with an ATU a "Kinda-Random Antenna" (KRA). (Apologies to linguistic purists.)

A simplified definition might be:  

  A. long enough to work within the maximum limits of the ATU's L-network on the lowest band used
  B. presents a reasonably low impedance on all bands used (e.g., doesn't look like an end-fed half-wave)

ATUs have limited monotonicity and granularity, as well as stray impedances, so in practice there is a third criteria:

  C. tunable on each band used despite specific L-network idiosyncrasies

This third criteria is the hardest one to predict for a given ATU design, as the idiosyncrasies vary with PCB layout and actual component values. They may only impact the highest bands, or for a particular antenna, the bands on which Q is the highest. For our ATU designs, we try to minimize strays and keep the network monotonic by using tightly toleranced capacitors and toroidal inductors.

While a wide range of wire lengths will meet the requirements of a "KRA" in the field, we've found from experience that something in the 25'-28' range works on all bands from 40 meters up, and roughly twice this for 80 meters up. Since it's impossible to predict the effect of ground losses, obstructions, deployed wire angles, etc., you may occasionally need to add or remove wire to obtain resonance on all bands used.

73,
Wayne
N6KR


On Jan 29, 2017, at 7:55 AM, Tom McCulloch <[hidden email]> wrote:

> I guess we need an alternate definition of "random" ;-)
>
> Tom
>
> wb2qdg

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[Matt Zilmer-3]

For general low-band operation, I use a doublet that is 75 feet long on
each leg, and fed via ladder line to a BL-2 balun near the shack.  The
fifteen feet of coax to the balun is low-loss LMR400.  The low-loss coax
helps with the high SWR problem, in that the losses are minimized (for
what I can afford, that is).

This antenna loads up fine on all bands except 6m.  I won't claim it's
ultra-efficient, but at least the ATU can handle it.

I chose the length of the ladder line as 61 feet, and this seems to give
good results.  I'm sure other lengths could be calculated that would
work equally well.

73,

matt W6NIA


On 1/29/2017 10:17 AM, Wayne Burdick wrote:

> I'd call an ad-hoc antenna that works on multiple bands with an ATU a "Kinda-Random Antenna" (KRA). (Apologies to linguistic purists.)
>
> A simplified definition might be:
>
>    A. long enough to work within the maximum limits of the ATU's L-network on the lowest band used
>    B. presents a reasonably low impedance on all bands used (e.g., doesn't look like an end-fed half-wave)
>
> ATUs have limited monotonicity and granularity, as well as stray impedances, so in practice there is a third criteria:
>
>    C. tunable on each band used despite specific L-network idiosyncrasies
>
> This third criteria is the hardest one to predict for a given ATU design, as the idiosyncrasies vary with PCB layout and actual component values. They may only impact the highest bands, or for a particular antenna, the bands on which Q is the highest. For our ATU designs, we try to minimize strays and keep the network monotonic by using tightly toleranced capacitors and toroidal inductors.
>
> While a wide range of wire lengths will meet the requirements of a "KRA" in the field, we've found from experience that something in the 25'-28' range works on all bands from 40 meters up, and roughly twice this for 80 meters up. Since it's impossible to predict the effect of ground losses, obstructions, deployed wire angles, etc., you may occasionally need to add or remove wire to obtain resonance on all bands used.
>
> 73,
> Wayne
> N6KR
>
>
> On Jan 29, 2017, at 7:55 AM, Tom McCulloch <[hidden email]> wrote:
>
>> I guess we need an alternate definition of "random" ;-)
>>
>> Tom
>>
>> wb2qdg
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--
Always store beer in a dark place.  - R. Heinlein

Matt Zilmer, W6NIA
[Shiraz]

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[K9MA]

There's an unavoidable tradeoff using an end fed wire with an ATU of
limited range.  Since the ATU can't match the high impedance of a half
wave multiple, the lower impedance of the wire will lead to larger
ground/counterpoise currents.  Even with a decent ground and/or
counterpoise, some power is lost, and at 100 W or above, you might need
to keep you fingers off the metal stuff.  That said, it can be a simple,
effective way to get on the air.

I've long been a fan of the EFHW, but I've always used an external
tuner.  The impedance should be somewhere in the 1K-5K range.  An
internal ATU might be able to match that with an external transformer,
say with a 3:1 turns ratio, though I'm not sure one could make such a
transformer with low enough stray capacitance to work on the higher
bands.  Has anyone tried it?

73,

Scott  K9MA

--
Scott  K9MA

[hidden email]

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[Hajo Dezelski]

Hi,

have a look at this one:
http://pa-11019.blogspot.de/2017/01/efhw-antenna-for-40-10m-qrp.html

It works as indicated in the measurements. I use it for the other bands
with the ATU of the Kx3 and Kx2.

73 de Hajo dl1sdz


---
Cela est bien dit, mais il faut cultiver notre jardin.

http://hajos-kontrapunkte.blogspot.de/
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