The overall frequency response is within +/- 1.5dB over the frequency range 1.8MHz to 50MHz.
Kent Britain, WA5VJB, suggested that I could further simplify my active antenna circuit, by omitting the complicated DC injection and regulator circuit and replacing it with a resistive load.
I took his advice and modified the output attenuator circuit to act as a load resistor and voltage dropper, as shown below. Note that the resistors used in the output attenuator network are required to dissipate some power, so 1/3 watt rated components should be used.
In this circuit arrangement the 0.1uF capacitor in the output attenuator network gives a slight increase in gain at frequencies below 100KHz, and the lower frequency limit is now 10KHz.
By omitting the ferrite beads and adding an additional HF equalisation network to the output attenuator, the upper frequency limit can also be raised to somewhere around 1GHz.
In the original circuit (shown above) I used BAT81 diodes to provide better input protection if the antenna is to be sited near transmitting antennas. These particular diodes have a low value of self capacitance, and do not affect the frequency response too dramatically. However when used in this configuration they can seriously degrade the intermodulation performance. An alternative protection input scheme using series connected, reverse biased diodes is shown below.
PA0RDT mini-whip voltage probe
This is a very simple compact antenna. I have built two so far. One is now working in the garden about 5m away from the house, but needs elevating higher off the ground than the current 1.5m. It picks up a lot of noise when close to the house.
all E probe antenas it is very prone to common mode noise. This is very
difficult to choke off the feed line due to the high input impedance of
the design. I may need to experiment and build a balanced version to
see if I can reduce noise pickup.
The frequency response of the version without the 10uH series choke connected between the antenna and the amplifier is very flat +/-0.5dB from 50KHz to 30MHz. Adding the choke lifts the response at 30MHz by approx. 10dB. The upper frequency limit seems to be constrained by the input capacitance of the amplifier. Modifying the shape of the antenna element to increase its self-capacitance improves the HF performance without degrading performance at the LF end of the operating range but more testing needs to be done.
Note that it's not a good idea to add back to back protection diodes across the amplifier input of these high impedance designs (as shown in the picture below). If you do want to add diodes connect them in series so that they are reverse biased between ground and +ve supply rail with the junction connected to the amplifier input.
I measured the IMD performance of this design (without input protection diodes) to be OIP2 = +75dBm & OIP3 = +32dBm
Owen Duffy has performed a detailed analysis of this antenna. Including the influence on it's performance by the length of feed cable and other factors associated with its installation.
There are also a number of kit verisons available if you want to make something that looks a bit more professional.
Since originally building this antenna, I have found another updated version designed by PA0NHC, who suggests that his design may offer better performance.
Prototype built on copper board on top of insulating layer of self-adhesive Kapton tape.
However the gain of the PA0NHC antenna seems to be about 4dB less than the PA0RDT design, which would further contribute to an improvement in the intermodulation performance.
graph below shows the measured gain frequency relationship of the two
types of antenna. In the case of the PA0NHC antenna I also experimented
with changing the value of the 0.1uFcoupling capacitors to 10uF in order
to decrease the lowest operating frequency. This would also produce a
similar result with the PA0RDT design if you are interested in the
reception of frequencies below 50KHz with these antennas.
that in order to achieve the reception of such low frequencies, the DC
injection / RF blocking choke in the antenna circuit and Bias Tee also
need to be increased in value from 470uH to 10mH. Ensure that any chokes used for this purpose have a low value of resistance and a high enough current rating. I used a FASTRON part number 77A-103M-00 10mH Inductance, 14.4 Ohms DC resistance and 300mA current rating, which is stocked by CPC.
The plots shown below were taken with 10mH choke inductors.
Orange trace = PA0RDT with simulated 20pF input load and using standard 0.1uF coupling capacitors
Red trace = PA0RDT with simulated 20pF input load and using 10uF coupling capacitors
Black trace = PA0NHC with simulated 20pF input load and using standard 0.1uF coupling capacitors
Grey trace = PA0NCH with simulated 20pF input load and using 10uF coupling capacitors
Pieter, PA3FWM has produced another updated Mini-Whip design based on PA0RDT's original.
This is now in use on the University of Twente WEB SDR http://websdr.ewi.utwente.nl:8901/
has produced a number of excellent active antenna amplifier designs. Some of which have been recently updated as a result of feedback from constructors such as myself.
I have built the circuit shown in Figure 5 in the first document.
This works very well and doesn't require any special transformers. Using
the component values shown in the circuit above, it has a flat
frequency response from less than 5KHz up to and beyond 30MHz.
The measured IMD performance was OIP2 = +59.1dBm and OIP3 to be = +43.8dBm which is
about 12dB better than PA0RDT.
So it is possible to use a slightly
larger whip antenna, which will provide improved S/N performance without
introducing a significantly higher level of intermodulation products.
I'm currently using this design for LF band reception on the SUWS Farnham WEB SDR.
This picture shows the antenna as installed at the SDR site.
The external housing is made from 50mm diameter plastic water pipe with 3D printed end caps and mounting clamp.
Clifton Labs Active monopole
Similar to one of Chris Trask's designs but with a few additional modifications.
I have one of these working on top of my works QTH (and another sitting around in my garage) that I use for making remote TX antenna field strength measurements. Although it's supposed to be a balanced antenna, it is not immune from common mode noise. However it gives very repeatable results, in terms of long term amplitude stability.
Chris Trask has analysed this design and produced his own circuit.
I've been building a broadband version of Chris Trask's remote tuned design. Which I hoped would outperform the Datong. However at the moment I'm having a few problems getting the circuit to function correctly. The individual stages work OK. But when they are connected together the frequency response is not particularly flat. A suggestion is that the cumulative phase / time delays through the cascaded stages. May be messing up the feedback circuitry. I've spent several days on this so far. So I've put it aside for a while and will revisit it later.
Cross Country Wireless HF Active Antenna
This is a relatively new design of balanced active antenna. Which is being produced by Chris Moulding of Cross Country Wireless.
I have an early production version, which works well. Performance is comparable to the Datong AD 270/370 (which is no longer available).
One interesting feature is that the RF 'head' and receiver interface are connected together via CAT5 multi-core Ethernet cable, rather than coax. Chris claims that this helps to reduce the incidence of common mode noise pickup on the interconnecting cable. I was not able to fully verify this. But the design did seem less succceptible to this problem than the Datong models.
I had previously built a version of the ARA-500 (50-500MHz) which used a fat printed circuit board bi-conical style dipole.
This used a broadband balun on the input of a BFT66 bipolar transistor RF amplifier with ferrite transformer derived negative feedback.
The circuit was similar to a broadband pre-amplifier design originally published in the German 'VHF Communications' magazine 1978 Q1 Pages 30 ? 36. Entitled ?A New Type of Pre-amplifier for 145 MHz and 435 MHz Receivers? By M Martin, DJ7VY.
The MuTeK BBBA 500u also used a similar circuit, but only using one stage of amplification. This provides a gain of about 10dB.
The later versions of Dressler such as the ARA-2000. Use FETs or broadband amplifier MIMIC's of various types and a strange helically wound antenna element constructed from foil tape.
Here's another analysis and a suggestion for an improved version of the Dressler
I'm now trying to get hold of a small quantity of RF3827DS RF amplifier chips to experiment with and maybe try building something like a better version of the Dressler Active antenna. This is proving to be somewhat difficult. So if anyone can help please drop me a line.
Lots of other active antenna links can be found here
SSB Electronics LNA-3000 Broadband masthead pre-amplifier (now discontinued).
Although not strictly an active antenna, this low noise broadband amplifier proved to be a useful addition to a Discone or Bicone antenna and often dramatically improved reception on the VHF and UHF bands.
Faulty units often appear on the surplus market, but it is not immediately obvious which semiconductor devices were used in the original design.
However I have successfully repaired units by replacing the two stacked semiconductors with MGF1302 Low Noise GaAs Fet's.
working correctly the source load resistor in the circuit has a value
of 33R (3 x 100R in parallel) which provides a voltage drop of 4.16v
with a bias current of approx 126mA for the two devices (63mA per
The amplifier typically provides a gain of approx 12dB, with a low noise figure and good strong handling performance.
Wellbrook ALA1530 and variants
I like the neat construction method that uses 15mm copper pipe and irrigation pipe tees. But I
thought that this is a very simple design would benefit from a more
recent type of amplifier chip. The PGA-103+ would be a good candidate to
try in place of the outdated INA-02186.
Tried a quick lash up using some RG58 test leads of odd lengths that were lying around to form a loop in the shack, and it works quite well. Especially considering it's sitting amongst all the PC's and noise generators. The loop size needs optimising, but I can already get quite good nulls and the loop isn't anywhere near properly balanced. I think it would work really well outside.
Although the antenna works to a certain extent there are a few problems.
The screened loop has several resonances within the required frequency range at around 4MHz, 20MHz and 40MHz which mess up the gain frequency response. It may be possible to shift these to less problematic frequencies by using an unscreened or smaller screened loop.
Another problem is that the PGA-103+ is designed for use as a wideband 'gain block' for use in 50 Ohm systems. However it's input impedance starts to rise at frequencies below 50MHz. So it's well suited for use in a 'voltage' probe E-Field antenna. But less suited for use as a 'magnetic' H-Field loop amplifier which needs to have a very low value of input impedance.
There is big mismatch between the loop impedance of low R high XC and 1K input impedance of the PGA-103+. It's possible to improve this by adding resistive damping to the PGA input, but it reduces the overall gain dramatically. I've also tried a 12.5 to 50 transformer which helps, but a lot of other equalisation would be required to get anything near an flat response curve. I suspect that this is likely to be a problem with the original design too.
amplifiers have an input impedance of around 2 ohms whereas the
PGA-103+ has an input impedance of around 1K ohm. So the PGA-103+ will
work as a loop amplifier, but the mismatch loss means that the noise
figure and gain flatness is not as good as it really needs to be for
Loop amplifier designs by Chris Trask and Clifton Laboratories are good sources for further inspiration.
One further line of investigation is to use two PGA-103+'s to form a balanced amplifier. One connected at each end of the loop with their outputs combined via a 0/180 phase impedance transformer to 50 Ohms. A bit like the G8CQK simple active loop shown below.
I've also thought about putting the amp in a die-cast box and attaching the ends of the coax loop with something like 'F' or TNC connectors. That way different size loops could be swapped as required or the loop could be easily broken down for portable / storage purposes.
Very easy to build. Reasonable noise rejection. 10dB variation in frequency response between 1.8MHz & 30MHz with peak at 10MHz.
Performance is reasonable but IP3 is quite poor. So only a small loop can really be used with it.
PE1KTH broadband loop
have now acquired the special op-amps that are required for this
design. But they seem to be very sensitive to mishandling. Especilly if
soldered by hand.
LZ1AQ - Amplifier for Small Magnetic and Electric Receiving Wideband Antennas
This is an excellent commercial product, model number AAA-1C. A very interesting design which can be remotely switched to operate as either an active loop or dipole.
Here's my crossed loop array using the LZ1AQ kit. The loops are 1m in diameter and made from LDF4-50 semi rigid coax.
The basic loop amplifier circuit is shown on LZ1AQ's webpage http://www.lz1aq.signacor.com/docs/wsml/wideband-active-sm-loop-antenna.htm
It's easy enough to build, here's my first version built on a bare PCB covered in Kapton tape, which I photographed whilst experimenting with different ratio output transformers.
I omitted the Input filter, 10v
regulator and used a large ferrite output transfomer (to avoid DC
saturation) with a coax feed and external bias Tee rather than using
The transistors are matched pairs of MRF581's which give a very flat frequency response, but 2N2222A's can be used for the input stage.
To get the best from this design you need to use a loop with as low a value of inductance as possible.
This is what I'm currently using. It's 1m in diameter and is 0.3m wide. It has an inductance of approxiamtely 1.6uH.
There is a wealth of useful design information on both of LZ1AQ's websites that are relevant for anyone contemplating the construction of an active loop antenna.
The DL4ZAO website also contains some useful circuits that are very similar to those of LA1AQ.
Note that the documents PDF copies written in German, so Google translate will not work. However it is still possible to understand the diagrams etc.
Ramsey Electronics - Signal Magnet Active Ferrite Rod Antenna
I like this as it uses a varicap tuned ferrite rod. Which has an electrostaic screen wrapped around it.
No circuit is included in the manual, but it's possible to figure it out from the PCB layout and Parts list.
Two of the best designs I've found are shown below.
Norton Lossless Feedback
Norton Amplifiers use a transformer to provide lossless negative feedback which linearises the amplifier and improves the intermodulation performance.
Most of the designs on the web are based on an article published by Dr. David E. Norton in the May 1975 Microwave Journal under the title, “High dynamic range transistor amplifiers using loss less feedback.”
Practical designs have been produced by Chris Trask and Dallas Lankford and detailed design information can be found here
Here's an example of a push pull Norton amplifier I've built using MRF581 transistors
Another type of design using conventional feedback techniques is based on the DX Engineering RPA-1 Pre-amplifier which is very popular for use with low gain antennas such as Pennants, flags, and K9AY loops.
The circuit has been reverse engineered by Jeff, F6AOJ, and published on his website http://f6aoj.ao-journal.com/crbst_236.html
My thanks to Jeff for allowing me to include this circuit diagram.
I built my copy using some MRF581 transistors which I had to hand.
The only modifications to the circuit being the addition of single hole ferrite beads to the transistor base connections and a reduction of C3 & C6 from 100nF to 1nF, which I found produced a flatter frequency response which was usable from 50KHz to 50MHz.
I also added some bias tee chokes so that I could remotely power the amplifier via the coax if required.
The intermodulation performance seemed to be slightly better than the previous Norton amplifier, and I found that it was a lot easier to construct.
This is my current preferred design for use as a distribution amplifier.