[Elecraft] [k3] Isotropic Antenna

[Fred Jensen]

On 4/6/2016 1:05 PM, Bruce Nourish wrote:

> I'm a little puzzled by your question, and I suspect others are too. The
> isotropic antenna is a theoretical construct which is sometimes used as a
> baseline for the comparison of other antennas; it is not possible to build
> one. The most primitive antenna that can actually be built is a dipole (but
> don't take primitive to mean ineffective!).

Just a wild guess ... he might have meant "Isotron?"
>
> Then I started
> reading about small magnetic loop antennas, built one for myself, and never
> looked back. I now mercilessly evangelize them to anyone who will listen.

I use an Alexloop with my K2 @ 5W for field operating.  It works better 
than the BP I had.  The only reason I sprang for the Alex [somewhat 
spendy as they say in OR] was that it was close to what I got for the 
BP.  You can homebrew one much cheaper ... but see comments on 
efficiency below.

The Alex is very easy to set up, takes me about 5 min, is light [~0.7 
kg] and fits in my pack well.  It is also very hi-Q and the BW on 40 m 
is barely wide enough for SSB.  It is very touchy to tune on the lower 
frequencies.  The radiation resistance, particularly on lower 
frequencies is minuscule, in the range of a few tens of milliohms. 
Thus, any resistance at all in the loop and cap can lead to very low 
efficiencies [e.g. use a butterfly cap to eliminate wiper resistance].

It is a resonant transformer and it is absolutely essential that you 
bypass any ATU and tune it exactly to resonance.  Fortunately, the W1 
wattmeter and the K2 both display SWR.  Getting it close and then 
letting the ATU do the rest results in a fairly expensive dummy load 
with a great match.

And a caution -- small magnetic loops can involve some surprisingly high 
voltages and circulating currents, even at 5-10 W.  They get into the KV 
range at 100 W or more.  Small mag loops are pretty much insensitive to 
ground and nearby objects since, unless you are set up in a railroad 
yard, the permeability of most things around us and the ground is close 
to vacuum permeability.

I sit under mine, I can reach up and tune it.  It has a bi-cardioid 
pattern, fairly broad forward lobes in the plane of the loop, and two 
sharp nulls orthogonal to the loop plane.  I never really mastered the 
BP tuning and, as a short loaded OCF dipole or vertical, its BW was very 
narrow too.

Antenna theory and practice are remarkably well defined [and close], and 
if someone offers you a "magic" antenna that's small and has the 
performance of 4 over 4 over 4 on 20, check and see if he also has 
ocean-front property in Colorado Springs for sale. :-)

73,

Fred K6DGW
- Northern California Contest Club
- CU in the Cal QSO Party 1-2 Oct 2016
- www.cqp.org