[?]Looking for an active antenna suitable for the home constructor.

-

Have you checked out Charles Wenzel's site?

Active Antenna

George H.

Why would you expect that such circuits would actually exist ?

Handling 1:10 HF frequency band with a single active antenna, might even be doable.

For MF/HF, you might try some whip antenna with a low capacitance FET source follower.

Below that, I would suggest some untunued (or preferably tuned) magnetic loop.

Doing a 1:3000 frequency range with a single antenna does not make any real sense.

Oh goodie, no numbers or specifications. That's the kind of design I enjoy best. I'll assume a receive only antenna, as a transmit/receive antenna is quite different.

You might try asking the same question in:

There are several good antenna designers that lurk there, and a few not-so-good antenna designers (like me).

Also check with the SDR (software defined radio) forums. SDR users tend to do things like digitize the entire front end bandwidth and then demodulate individual signals at their leisure. The need wideband antennas to do that.

You should care much about front end performance below about 7MHz, where atmospheric noise is sufficiently high that a more sensitive receiver will just hear more noise.

One major concern you should have is overloading the front end or subsequent stages. Broadband receivers are by definition, broad as a barn door. The FET front end amplifies everything between 10KHz and

30MHz at the same time. It's very easy to turn your receiver into a mixer. If you happen to have a very strong station nearby, your FET will either mix well and serve you intermod products, or overload causing the receiver to desensitize. Spend some time calculating the gain distribution and overload points of both the preamp and the receiver. If done correctly, all stages should overload at the same signal level.

Yep. If that's what you want, you might be in luck as the JFET was usually a source follower or cascode arrangement, intended only to drive a 50 ohm coax line from the high impedance antenna. Since there's no gain, all you have to do is make sure that the JFET can handle the highest expected input signal without overload.

However, you don't need a FET as it can be done with bipolar devices:

Sure. Forget the loop antenna, which is only useful for limited space installs. Setup your favorite long wire antenna. Measure it's impedance across the frequency range. Built an antenna tuner with memory (as in LDG Electronics) and program it to match the long wire to 50 ohms.

Nope. MMIC's also like to see 50 ohms on the input. In order to keep them happy (and possibly stable), you'll need a matching circuit between your antenna and the MMIC input. Might was well stay with the JFET source follower or maybe cascode.

At 30MHz and below, you can build it on a wooden cutting board (the original breadboard), using nails for tie points, and wrapping the leads around the nails. SMT would be nice, but not really necessary.

Antenna design software:

I suggest 4NEC2 which comes with a huge collection of sample antennas suitable for plagiarizing.

Build it on paper first. Do the math. Then construct it. If it doesn't work on paper (or computah), it probably won't work when you build it.

After you build it, some way to test and measure your results will also be helpful. A VNA (vector network analyzer) would be nice, but an MFJ259 or MFJ269 will suffice.

If cheap, build or buy a reflection coefficient bridge:

This will give you the impedance magnitude, but will not tell you if it's inductive or capacitive. You'll also need an RF sweep generator to use it.

Good luck.

--
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558

You might explain why you want such an amplifier. My first thought is that you would want to have a bunch of band pass input filters so as to avoid over load from strong stations no in the frequencies you are interested in. My second thought is you might be better off with a bunch of amplifiers each with appropriate band pass filters and notch filters for the in band strong signals.

My third though is you go back and think about why you want such an amplifier.

Dan

David Chapman wrote:

Many thanks to those who have already replied to my posting and for the various web-site references mentioned. All very interesting and potentially useful.

Let me try and explain my requirement in more detail:-

I have an AR5000 receiver which I use for monitoring LF through to UHF but close to the house there is a very high level of electrical noise in the bands up to around 30MHz.

This noise considerably as I move further away from the house so, for receiving these lower frequencies I'm planning to mount a suitable wide-band active antenna, with 1 metre long rod antenna, around 150 feet away from the house and power it over the same co-ax feeder that brings its signals back to the receiver.

I'm hoping to find an amplifier design that has very good dynamic range to cope with a few strong broadcast signals, and assume that this means that it will need to draw a current of some tens of milliamps rather than the few consumed by some of the most basic designs that I've seen.

I've no serious interference problems with the antennas that I use for frequencies above 100MHz - it's only reception of the lower frequencies that I wish to improve.

I hope this explains more clearly what I'm trying to achieve.

- Dave.

--
David C.Chapman - (dcchapman@minda.co.uk)
--------------------------------------------------------------------------

A very short antenna is very reactive, in practice the capacitive reactance of a small (10 pF) capacitance. Thus, it is essential that the amplifier input impedance is as high as possible. With FETs, the input resistance can be made very high, but the problem is the capacitance from input to ground. Together with the antenna capacitance, this will form a capacitive voltage divider.

If the amplifier input capacitance is the same as the antenna capacitance, there is going to be a -6 dB signal voltage loss.

A small magnetic loop will have a very low radiation resistance (milliohms or even micro-ohms), thus the amplifier impedance should have as low input impedance as possible. In that link, a grounded base configuration is used. To reduce the impedance even further, multiple small signal transistors could be used in parallel or even some power transistors (e.g. in TO-220 case) could be used.

For proper operation, bandpass filters usually require quite well defined source and load impedances. A short whip will have a highly reactive impedance varying with frequency, so this must be considered in filter design. Not any text book filter will do.

Might it not be worth doing a careful survey of your household appliances first with a handheld receiver and then putting better RF suppression on the worst offenders? Likely to be electric motors, dimmers and various other kit that arcs and sparks.

That has always been the approach taken at radio astronomy sites.

You will be better off notching out the strongest local signals.

Regards, Martin Brown

Why not simply build separate antennas for each band with dedicated amplifiers for each antenna and use relays to switch between the antenna/amplifier combinations.

For the lowest bands, you could even use motor driven variable capacitors in magnetic loops at the lowest bands. Switching out some of the loop turns with a relay, at least a 1:10 tuning range should be doable.

If you are using remote antenna locations, please remember to use some ferrites around the coaxial cable near the house, in order to avoid any interference currents flowing on the outside the shield (which could then reradiate into the antenna).

[...]

Can you localize where the noise is coming from? Seems to me it'd be easier to fix the noise source...

Two problems there:

Whilst I can locate and (probably) fix any interference generated by computers and other equipment actually in this house, some is coming from houses to either side of ours. This QRM is much harder to pin-point exactly and MUCH more difficult to fix without spending a lot of time and dealing with a lot of unwanted hassle with neighbours..

....and secondly, the reason I'm looking for a physically separated active antenna using a short (1 metre long?) rod is because I don't want to go the route that involves the erection of unsightly long wire antenna(s) adjacent to the house.

I KNOW that an active antenna will do the job for me since I've tried one temporarily and it works very well when positioned more that 100 feet from any nearby house and connected to the receiver by co-ax.

Before deciding to build a design that was produced 10-20 years ago I thought I'd ask in this NG to try to find out if modern semiconductors have enabled any worthwhile performance improvements for this application.

Maybe today's technology has nothing more to offer in this particular application of course, in which case I'll just build up one of the original J309 FET designs, but I thought it would be worth asking the question at the very least.

Thanks again for all the interesting comments and suggestions.

73 - Dave.

--
David C.Chapman - (dcchapman@minda.co.uk)
--------------------------------------------------------------------------

Ferrite rod? I've never seen one that is 1 meter long.

I think you're fighting the wrong problem. You need to at least identify the sources of the RFI/EMI noise. Some of the common problems are BPL (broadband over power line), switching noise from solar power systems, plasma TV screens, big motors, etc. Around the house, the new switching power supply wall warts are really awful. Computers, computer networks, and laser printers are also RF generators.

etc... Google for "identify rf interference sources". There's also some stuff on YouTube with examples of noise sources.

Building the ultimate receive antenna isn't going to help if the sources of RFI/EMI are constantly changing and moving. The best you can do is probably to build something with some directionality, so that you can aim it away from the worst RFI sources.

As for antennas, bigger is better. Also, the uglier it looks, the better it works. The problem is: Gain, Size, Bandwidth.... pick any two. With a small broadband antenna, you're not going to get much antenna gain.

--
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558

Hi Dave, Send me a private message to snipped-for-privacy@knology.net Be sure to removethis. I couldn't get your address to work. I have some info for you. Mikek

No, not ferrite (I've never seen one that long either) but just a straight length of stainless steel rod around 2-3mm in diameter.

Yes agreed, and I fully accept that your proposes course of action is certainly the correct scientific way to resolve the interference problem, but I really don't have the time or inclination to spend hours locating and sorting out problems that I already know can be largely overcome by using an active antenna 100 feet or so from the house and running a co-ax feeder from it to the receiver indoors. Life's too short!.

Understood and accepted, but I don't need gain, only Hi-Lo impedance transformation and a 50ohm co-ax driver. As I said earlier, I know that an active antenna will work sufficiently well to meet my needs - I just posted to this NG in order to ask the experts if there have been any significant improvements in semiconductors during the last decade or so for this application.

Having read and digested the various interesting and informative responses to my posting I'll now probably go ahead and build an active antenna to one of the earlier designs since I've got lots of J309 and Siliconix E310 as well as E430 (dual) J-FETs in my spares box.

Many thanks for the references to useful reading on other web-sites.

ATB - Dave.

--
David C.Chapman - (dcchapman@minda.co.uk)
--------------------------------------------------------------------------

Well, ok. A 1 meter steel rod at 3.5Mhz (80 meters) would be umm...

1/80th of a wavelength long. That's a rather small/short antenna. I think the telescoping whip on my Zenith Transoceanic is longer. The amplified Loran antennas I used to play with were about 10ft long.

If it were a dipole, it would still have about 2dBi gain down to about

1/10th wavelength long after which it drops fast. I'm not so sure about a monopole but I suspect it would be similar. If I have time (unlikely), I'll throw together a 4NEC2 model demonstrating what a mediocre antenna a short monopole can be.

I suspect that the only reason the antenna doesn't hear the EMI/RFI from the house at 100ft away is because it has no gain. Similarly, it's unlikely to hear any other stations.

As for not enough time, I'm fairly certain that there will be plenty of time available to do it correctly after you've wasted time working on the band-aid solutions. EMI/RFI problems are best fixed at the source.

I suggest you repost your question to news://rec.radio.amateur.antenna which should offer some better help.

--
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558

If this antenna would be used for transmitting, it would require a large (and lossy) base loading coil, in order to present a resistive

50 ohm load to the transmitter. The efficiency might be below 1 % and the bandwidth perhaps only 1-2 kHz.

However, the OP is talking about a receive only antenna and since the background noise level is quite high at this frequency (not to mention VLF/LF/MF bands), so the efficiency is not an issue. Please remember, a typical MF (AM) ferrite rod antenna might have a gain of -50 dB (0.001 % efficiency) and still you are able to receive broadcast stations. With a voltage follower after the highly capacitive antenna rod will make the system quite broadband.

The problem with unmatched active antennas is the stray capacitance between mains wiring and the antenna element. Any interference voltage in the mains wiring is easily coupled to the antenna element.

Loran-C is at 100 kHz i.e 3 km wavelength, thus, the element would be

1/1000 wavelength.

What is the stray capacitance from the house wiring and the antenna at

30 m ? I guess it is quite low and hence the capacitively coupled near field interference is quite low.

Please state clearly that it is a receive only application, since otherwise most readers would make suggestions about tuned narrow band Rx/Tx.

If you are interested in frequencies below 300 kHz, it would make sense to check the various LowFER pages.

Yes, my requirement is definitely Receive only - my AR5000 receiver doesn't transmit at all well ;-)

- Dave.

--
David C.Chapman - (dcchapman@minda.co.uk)
--------------------------------------------------------------------------