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

I'd be most grateful if someone could point me towards a suitable
design for a 'roll-your-own' active antenna, with high dynamic signal
range, suitable for use over the frequency range 10kHz to 30MHz (or
above).

I've been looking at the Avago range of FETs which have excellent
performance in the GHz region with good dynamic range, but I have heard
that they do not perform as well down in the KHz/MHz frequency ranges.

Most of the active antenna designs that I've seen for the lower
frequency bands, admittedly published in old ham-radio magazines, seem
to use a J309 or equivalent J-FET as a front-end device.

Before starting to build up one of these old active-antenna designs, I
was wondering if anyone in this NG has any recommendations for a modern
'2012'version with (possibly) better performance that would be suitable
for an enthusiastic amateur to build. Maybe one of the higher-current
wide-band MMIC amplifiers would now be better suited to driving the 50
ohm output than the discrete component circuitry used in all the earlier
designs that I've seen.

I'm quite happy designing/building PCBs and I regularly work with SM
components down to 0603. I don't have any professional-standard design
software (or the ability to use it) but I do have a decent signal
generator and spectrum analyser ready to hand.

Any constructive (!) suggestions will be gratefully received.

TIA - Dave.

 

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.

 

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.

 

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.

 

Hi Dave,
Send me a private message to
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.

 

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.