What exactly would be the problem with a small amount of ferrous material inside a QFH GPS antenna?

Question

This video describes the construction of a home-made QuadriFilar Helical (QFH) antenna for a GPS receiver module. A few seconds after 04:00 the narration says:

Being a loop antenna I’d imagine it unadvisable to use any ferrous material at all, including the braid of the coax which is running straight through the whole antenna.

With large diameter conductors, no tuning instructions, and no ultra-critical dimensions in the fabrication, I'd imagine this is not a terribly high-Q antenna. Early in the video the design wavelength is listed as 190mm - the 1575 MHz L1 band of GPS.

What would be the primary problem (if any) caused by a small amount of ferrous material inside this particular antenna design? A shift in resonance frequency, or unwanted nulls in certain directions of the radiation pattern, or just an overall loss of gain?

Answer 1

I wouldn't worry about ferrous metals near the antenna. As you suspect, it's probably a bit of superstition carried over from low frequency loop antennas, where it's more important.

First, the QHA is a balanced antenna, so there shouldn't be any significant currents flowing on the feed line in the centre

Then, it's not as high Q antenna, so won't suffer unduly from a more lossy material. In fact a GPS antenna won't notice a fair amount of loss - the typical little square patch antennas are less than 50% efficient, and the linear polarised chip antennas used in cellphones, for example, are even worse.

Finally, steel and stainless steel get a bad rap in antennas. They're not good for making very short or small transmitting antennas, with high Q and large circulating currents, but on a resonant dipole, even at 7 MHz, the loss isn't too bad, considering the strength, cost and corrosion improvement over copper. At higher frequencies, a lossy material in the antenna matters even less.

You should worry, though, how the QHA is fed - it needs a balanced feed. There may be a sleeve balun lurking in the brass pipe. I haven't seen the video. If it's just fed directly from the coax, the patterns won't be ideal.