Spiral antennas are funny devices. They are known for their extremely wide bandwidth, the ones shown here, are specified to operate from 2-18GHz.

They’re not meant as transmitting antennas, these critters are receive-only, used by (largely non-civilian) pilots who have those needs

Their low frequency boundary is determined by its physical size, in this case, the circumference of the small plate where you can see the fine spiral traces.

Once you cross below that boundary, the structure basically collapses electrically, seen here as a steep SWR ‘wall’ to the left side of the sweep.
At this point it’s not an antenna anymore, but something else (whatever that is )

Always wanted a portable directional to spot emitters at higher frequencies - decided to try this one.

My test ceiling is 4GHz, but at least that far, it seems OK I doubt it’ll go all the way to 10GHz as advertised because of the (FR4?) PCB Material.

It always amuses me, how violently antennas like this collapse at the lower frequencies - suddenly the size no longer accommodates the input wavelength and they simply stop being an antenna

For some reason, these logperiodic (LPDA) antennas always come with a SMA connector at the tip - that’s completely wrong: you need to solder a semirigid coax all the way down the centerline and feed it from there. The PCB was even prepared for it - go figure

Anyway, now fixed, tested, and ready to go signal hunting

Likely of most interest to hamradio operators, but maybe also for others sharing tower real-estate with powerful emitters

Upon installing a 868MHz antenna, I found it to also have a response at the (hamradio) 145MHz band

I decided to test how much power it would pick up from my nearby (6m separation) VHF/UHF antenna when operating the transmitter at different output power levels.

I found, that at 145MHz, the full 50W power of my FM transmitter would produce short of 14dBm at the antenna connector of the LoRa module which is above the maximum ratings of the widely used SX1262 transceiver chip.

So be careful if you’re close to powerful VHF/UHF emitters, the SX1262 is wide open from 150-960MHz and since the onboard filtering is likely rudimentary to cut cost, you may need an external filter to protect your module.

Have a nice, EMI Safe, weekend out there

It’s #testgeartuesday and here’s a fun and easy all-time classic antenna experiment: measuring the near field strength across a half-wave dipole

Cut a dipole for a suitable frequency, say, 145MHz. If you have a small VNA, use it to verify the center frequency.

Now, feed the dipole at the center frequency with an external generator, here a vintage classic: the #marconi 2022 Output power was set to 10dBm, that’s 10mW, more than enough. Don’t worry about walls and reflections, it’s the near field we’re interested in.

Carefully moving the field strength meter across the dipole, from one end to the other, will show the voltage curve: maximum at the ends, zero at the feed point.

Such a simple experiment - demonstrating such a fundamental antenna behavior

It’s #sensorsunday

A field strength meter can be really handy when testing antennas and they’re simple to build: a couple germanium diodes, a capacitor, a good old analog uA meter and a potentiometer to adjust the sensitivity.

This unit was just $20 and it seems they repurposed a PCB originally intended for some other application

To use it, start by adjusting the sensitivity to its minimum setting, key the transmitter at its lowest power (1W will certainly do) and walk along the antenna and measure the field strength along the wire or structure.

I’ll do a future post demonstrating it on a trap dipole , W3DZZ style

Log periodic antennas are wide-band directional devices, suitable for signal hunting or simply wide-band communication. Their gain isn’t impressive, this one is rated to about 6dBi, that’s the price you pay for bandwidth.

The commercially made are really expensive, literally an order of magnitude above this eBay unit which was about 60$. But I suspect they have a similar PCB inside anyway

So how well does this unit perform? My #nanovna_v2 cuts off at 4GHz but starting at about 700MHz, it surely delivers a reasonable match below SWR 1:2 (620MHz for SWR 1:3)

I’ll have to bring it to the lab at work to test the upper frequency limit For now, let’s go signal hunting!