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Active matching circuits on low frequency antennas

Active matching circuits on low frequency antennas

Active matching circuits on low frequency antennas

Has anyone experience with active matching of low frequency electrically small antennas (low freq = 0.1-50 MHz ballpark) (electrically small, less than 0.1 wavelengths).

Called non-Foster matching circuits, op-amps are sometimes used to make negative capacitors and negative inductors to tune out low frequency monopoles that are electrically short and look capacitive with low radiation resistance (0.1-4 ohms). I've read a few papers that had measured data from 1-30 Mhz, but haven't seen any real hardware.

Anyone have experience?



RE: Active matching circuits on low frequency antennas

Google "Active Antennas". They're typically well-insulated whips that feed directly into the gate of a FET. They've very common when people want to listen to HF, MF, LF, VLF, and ULF and all the way down.

I own three or four. They work very well, but obviously have no directivity. Sony AN-1 I think, and an MFJ desktop unit. Perhaps others, I've lost count.

By well insulated, it means that the antenna element needs to be kept well away from anything, and brought into the circuit with a Hi-Z single wire. You do NOT use coax between the whip and the FET gate.

The above assumes that you're asking about receive only.

For transmit, the issue is incredibly low feedpoint Z which calls for insane wire gauges between the transmatch (often an autotransformer) and the antenna. Hams that do 160m (1.8MHz) and 80m (3.5 MHz) mobile - using a 9 to 12 foot whip) would know all about this.

I've never bothered to totally figure out why the short antennas are low Z at transmit, and treated as ultra-Hi Z for receive. I think it is because they treat the whip as a Hi-Z voltage probe on receive.

You could also see what the ULF'ers or LF'ers are doing. Some of them build contraptions that can receive right down into the single digit Hz.

I guess in summary - some people don't worry too much about matching for receive, but they get the job accomplished using another approach.


RE: Active matching circuits on low frequency antennas

Thanks VE1BLL, I've heard of these FET matching devices. Do you know how much sensitivity improvement is offered by them?

I'd guess that the FET producst you mentioned maybe a narrow band match, comment please? Do they sell one unit that covers all bands or do they make you buy a separate unit for each band?

I'm looking for an ultra broadband active match.


RE: Active matching circuits on low frequency antennas

Generally, the noise floor is external and thus you can hear signals right down into the noise floor.

They are of course omnidirectional. Serious LFers use directional antennas, or nulling schemes.

Most of the commercial products will cover xx kHz to 30 MHz. xx might be 10 kHz or 150kHz (depend how ambitious the designer was). But many will have a selectable high pass filter to reduce overload from nearby AM stations.

The circuits are next to nothing. The whip feeds the gate straight in. The FET needs to be at the base of the whip. This part of the circuit needs to be treated as multi-Gigaohm for best results. Keep it up and away from all grounded metal.

You need to feed power out the coaxial cable to power the remote FET circuit. Once you're past the FET, then you can use normal cheap coax. PS filtering is another thing to watch for - I normally use batteries to skip the humm issue.

There are people monitoring the Earth's 8Hz field using similar antennas and their PC's sound card.

My Sony active antennas are the AN-1, and the other one is the tunable desktop MFJ-1020A (not recommended). The Sony AN-1 is outside on a stake and I can listen right down to where my SW radio gets funny (~30 kHz or so).

You can Google up a schematic and get the concept. Then compare it to any other options. Good luck.

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