Recommendation for an antenna

[nanZor]

Well now you've really done it. Just when I thought I was beginning to catch on you tell me the only part of the antenna the R75 is using is the radials that are attached to the ground shield of the coax. If the shield is grounded, how is the antenna working at all?

Coax is a 3-wire circuit to RF. Two of those wires are the outer skin of the braid, and the inner skin of the braid. RF travels on a very thin skin-depth of a conductor, unlike DC which uses the entire width of a conductor. The 3rd wire consists of the outer skin of the center conductor.

Current traveling on the outside skin of coax is known as "common mode" current. It has no equal-and-opposite current flow since it is on the outside, unlike the inside of coax where the current is known as differential aka transmission line current where there IS equal and opposite current from the inside of the braid facing the outside of the center conductor.

You can make your own common-mode random wire by connecting the center conductor of a piece of coax to the shield at the far end, and at the receiver, only connecting the center conductor and leaving the shield free/ungrounded. That's a lot of work and waste of coax to emulate a regular piece of wire! However many snake-oil antennas, like the coax ground-snake or others that use a similar technique work.

Grounding isn't a guarantee that rf won't flow on the outside of the coax braid skin. See the End-Fedz FAQ for "Do the EndFedz require a ground?" that touches on the subject. A more complete treatise on end-fed antennas and the effects of common-mode and grounding can be found here:

http://www.aa5tb.com/efha.html

Many antennas rely on the outer coax braid to complete the circuit. In fact, the so-called "magnetic loop" that only responds to magnetic fields only does so when within a few feet of an rf source. Otherwerwise, they too use "common mode" current on the outside of the loop conductor to act as the antenna. An HF rated ferrite clamped around one of the arms of these loops will physically prove it by destroying the nulls when 50 years of misinformation doesn't. If these were truly responding only to "magnetic" fields, the choke shouldn't have any effect - but it does. It is easy enough for anyone to prove to themselves with a loop and a good ferrite choke.