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[KC1SOZ]

Hi Folks, just getting started with the ham radio hobby, taking my technician and general tests next month. I'm gathering gear, including a Yaesu FT-840, an FC-800 autotuner, power supply, etc.

Today I went to see a local seller who was selling gear from someone who moved to Florida and could not take all his stuff with him. Some of the things i picked up I easily recognized, but this one has me stumped. It seems to be a 6 meter quarter wave dipole with a center box that probably amplifies the signal with a "K332" Silicon N transistor (2SK332). The dipoles are 77 cm each, for 154 cm, which is just slightly over the size of a 6m quarter wave antenna.

Does anybody recognize this funky little thing? No markings, of course, but excellent condition.

[WB6BYU]

Given the circuitry inside, I'm guessing that is a wide-band
HF receiving antenna, although that coil at the top center
of the photo might peak it at a specific frequency.

The DC voltage to power the amplifier probably is run up
the coax.

If nobody else recognizes it, we could take out the circuit
board and trace the circuit to see what it does.

The use of the red toroid cores with bifiliar (two-wire)
windings would tend to suggest it was designed for
VHF or the upper HF bands - perhaps FM broadcast,
in which case the adjustable coil might be a trap to
reduce a particularly strong station that overloads
the amplifier.

[KD7RDZI2]

Do not transmit into it and do not power it while transmitting. You would burn it. It is an active hf antenna somehow like the Datong Active dipole. It could perform very well and the fact it is a dipole rather than a monopole could not pick up local noise. So, do not destroy it! In case It would not work, replace the transistor. It looks serviceable. Don't underestimate it could be a well designed active HF antenna for receiving purpose. It might work well from few khz, maybe 10 khz to maybe 50 Mhz. But it needs to be powered via the coax cable with a bias T sort of interface between the receiver and the coax (a capacitor, a inductor and maybe a resistor) to provide the right milliamperes at the right voltage.

[KC1SOZ]

These comments make perfect sense, thank you. But I think I'm going to be really stumped trying to figure out how to power this. It seems it requires a control box to feed it the power the transistor and coils need to do its work. The Datong uses a 12v power supply, but without some identifying marks I am not sure I want to experiment with my Yaesu.

I am going to remove the board from the box to see if there is anything on the trace side to tell me who made it, model #, etc. If nothing else I can always take a good photo and someone can try reverse engineering it.

[KD7RDZI2]

I would not dismantle it. The control box should be very simple. It is a Bias-Tee you find for few dollars on amazon or ebay. The inductor in the bias tee serves to filter the (non switching) power supply. Larger the inductor the better for VLF and MW. A resistor might be needed to limit the current but it might be already in the unit you have. So it might almost work plug and play. The required voltage might be 12v but looking at the picture is impossible to determine with certainty. The Datong was working between 11 and 14v dc. These antennas are very good for receivers from the vintage to the most modern SDRs.

[KC1SOZ]

So I am looking at the bias tee products, and I saw the MFJ4116. But I see that there is an injector and an extractor. Can I assume correctly that in this case I only need the power injector and not the extractor because I am not intending to want to get power out of the antenna for some other accessory?

[KD7RDZI2]

I meant something like https://www.amazon.com/Ciglow-Amplifier-100MHz-Wideband-Coaxial/dp/B084MFKQ14/ref=sr_1_7?crid=31MY4H7O0DEZ8&keywords=Bias-Tee&qid=1677698339&sprefix=bias-tee+%2Caps%2C879&sr=8-7

or similar things just to give an idea. On Amazon there are plenty. Just pay attention to the lowest frequency so it can work well in VLF and MW other than HF and VHF. Active antennas like yours should shine on lower bands and work well at least on HF.

You need just 1 bias tee. The bias tee will provide the voltage to the antenna through  the coax cable. The manual of the MFJ unit is not very clear for your type of application. The MFJ may provide up 1 Ampere but your antenna will draw much less, likely around 0.1 Ampere.

[KC1SOZ]

What frequency range should I be looking for for best performance?

[KD7RDZI2]

The one I have indicated should work from 25 khz up 100 mhz and that should be enough for your active antenna. I would avoid those for VHF, UHF and SHF that say from 10 Mhz or so. But that depends on the receiver you wish to use. The DC filter inductor for lower bands is bigger, you see it very easily, sometimes a toroid is used. The capacitance of the capacitor to block voltage to the receiver is also higher but the dimension of the capacitor may look small. If you select one just post the link. Someone can give you an opinion if it might work well.