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 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
