Seeking oldest dual-band non-uniform monopole helical antenna 1

[roger88]

I am seeking the earliest example of a single-conductor helical antenna with two different sections (differing by either pitch and/or diameter) designed for dual-band (or multi-band) use - particularly in the normal mode.

 

[VEBill]

'Normal mode' of course means a plain old loaded whip antenna (as opposed to 'axial mode' as typically used with satellite comms).

I suspect that technique is as old as the hills. Certainly very common in the late-1970s, early 1980s for multiband VHF/UHF scanner monopoles intended for mobile use (on cars). Radio Shack certainly sold such VHF/UHF whips with two loading coils.

Going back even further (say 1950s for example), there is less UHF and more HF & VHF. The typical VHF or UHF radios of the era wouldn't have required multiband antennas since the radios were typically not multiband capable.

Therefore, you'd probably be looking for either amateur radio or military applications in the upper end of the HF range or low VHF. Military typically wants ultra wideband (fan dipoles and various cones), but amateurs might want to cover several ham bands (20m, 15m, 10m) with one whip. But they often used traps (coils with capacitors) instead of just coils back then.

But even the Radio Shack VHF/UHF whips from the 1970/80s should be sufficient to bust any current patent (if that is what you're up to). I'm not sure that the two coils were different. but I think that they were (I remember the top coil was smaller than the bottom middle coil). And I'm not sure that they were just coils (no caps) either, but I suspect that they were just coils.

Also, I'm pretty sure that I've seen a larger 'rubber ducky' antenna (the black rubber with a BNC, or threaded screw, on the bottom) with a variable helical at some point along the way. Probably came with an early portable scanner from Radio Shack.

In summary, 1970s/1980s Radio Shack scanner antennas should be a good hunting ground.

 

[VEBill]

US Patent 4,229,743 is almost exactly what you describe.

http://www.freepatentsonline.com/4229743.pdf

Being from 1980, I assume it is expired.

 

[BrianR]

I built my first normal mode multi-band helical in 1970 and they were in comercial use then. I can recall examples from around 1967 onwards. My antenna used fishing rod blanks with a standard base section and replaceable tops. The base section was wound with very heavy wire as it carried most current and thinner helical windings on the top sections.

 

[roger88]

BrianR,
Are you saying that you had a single wire wrapped tightly at the bottom and not as tightly at the top such that you could receive two different non-contiguous frequencies?
What do you mean by "replaceable tops?" I'm not quite picturing this design.

-Roger88

 

[BrianR]

This was a simple design and I replaced the top section to select a second band. I could have used in-line tuned traps by resonating some sections of the continuous winding and achieved multiband that way but that's not how I did it.

I chose to wind the common base section as loosely as possible using heavy wire in order to push the current lobe as high up the antenna as possible and reduce losses. The top section windings were pushed as high up the element as they would fit too and in each case I used the thickest wire that would fit. A corona cap is needed to prevent corona at lower frequencies (size depends on the power you run).

Building it this way minimises copper losses and improves the antenna efficiency at the expense of VSWR. However as a short vertical at HF has a VERY low radiation resistance then the VSWR should be poor at 50 ohms and a matching transformer is needed. Some comercial units avoid this by using lossy construction and increasing loss resistance to reduce the VSWR at hte feedpoint but I have seen some of these catch fire too.