Why the old formula of 1005/freq sometimes doesn’t work for loop antennas.
It’s been a well published fact over the years, that the formula 1005/freq [that’s 1005 divided by the operating frequency] will give you an accurate full wave length of wire [in feet] at that said frequency.
It’s great for building quads and delta loop antennas or any other loop for that matter. Given that this formula works out the length in feet, we’re presuming it came from the US as it’s mentioned in quite a few of the older North American based antenna reference books of the 1960’s and 70’s.
To test the theory, we cut a section of 2mm bare copper wire using a frequency of 28.450MHz. The formula gave us 35.352 feet which when converted to metric gave us a total length of 10.767m.
We tested this on a 3 sided single element delta loop with each side being 3.589m and hung off on non-conductive fibreglass pole and tied quite crudely to some fence posts. Nothing here that’s rocket science, only that there wasn’t any ‘metallic’ objects in the near vicinity to upset the test.
Checking with an MFJ258B analyser saw the predicted dip at 28.440MHz – so pretty well spot on.
However, I had seen some ops mention that they had poor experiences with some homebrew loops and couldn’t understand why they seemed wrong and didn’t work as expected. This became more of an issue with builders of multiple-loop antennas as if ‘one’ loop was [wrong] – then they could ALL be wrong.
If this was the case – then potentially you could have your nicely built homebrew quad or delta loop firing out the back-end or having poor bandwidth/gain or a mixture of all three!
The issue seemed to be when using PVC coated wire. PVC or ‘coated wire’ is often found cheaper than bundles of bare copper wire [well it seems to be the case in the UK] so as we’re all looking to build something as cheaply as possible, coated wire was used and whilst physically there’s nothing wrong in using it, builders must be aware that the PVC coating alters the velocity factor of the wire.
The exact amount can differ depending on the coating, but a good general rule-of-thumb tended to be a velocity factor of 95% [for PVC Coated wire] when compared to bare copper.
So, what does this mean to our calculations. Simply put, when you have your ‘base’ figure, multiply it by 0.95 for PVC coated wire [or 0.96 if the coating is Polyethylene or 0.98 if it’s Teflon/PTFE]
As an example for our current loop, we have a circumference of 10.767m. If we were using PVC coated copper wire, then the length of the loop to obtain resonance around 28.450MHz would be 10.767 x 0.95 = 10.228m. so quite a difference and a loop that’s nearly 54 cms smaller in circumference.
In essence, coated wires reduce the velocity factor of the RF meaning that for a given frequency, the loop would be smaller than if bare copper wire was used.
It’s a similar story for other antennas too – so always worth remembering.