Active antenna project


Currently we have: a double loop,  40 mm PVC pipe, 4mm diameter copper wire, loop diameter 1 meter, vertical dipole:

Figure 1: copper wire painted black
Figure 2: Install the amplifier, check the jumpers
Figure 3: This is what it currently looks like, it is 12 meter behind the house
Figure 4: Inside of the antenna control box.


Imagine that you are in a busy pub trying to understand your friend 10 meter away from you. Usually this does not work unless everyone else in the pub stays silent for a second. For shortwave radio amateurs this is a very common problem, man-made noise is all around us and you can’t easily turn it off. (In a pub you can’t either) But what you can do is to put your hearing aid, the antenna, further away from the local noise, this is precisely what an active antenna does.

Antenna’s for shortwave radio (frequencies up to 30 MHz) have two contradictory properties, their size is ideally half of the wavelength but a half wavelength is already between 40 and 5 meter. This is a requirement when you transmit, but, if we are only interested in listening to a shortwave radio then a long antenna can be a nuisance because it is more likely to pick up man-made noise than a smaller antenna does in an urban environment. Ideally you would like to stay something like a wavelength away from all other electronics inside your and your neighbors house that causes man-made noise.

For this reason a lot of radio amateurs use websdr receivers such as at the university of Twente, its antenna is in a quiet environment, it is a mini-whip antenna, thus an active antenna. The only problem is, you need internet and also many amateurs dislike the websdr because it is not their own receiver. For this reason I wanted to have my own solution, the idea is, find a quiet spot around the house and put up an active antenna.

Over the past two weeks I’ve been busy installing the amplifier board that I got
from I’ve chosen for a double loop antenna on a 2.7 meter pole at the end of our garden, the loop diameter is approximately 1 meter. The amplifier has several control lines, jumpers, it needs 13.8V and it generates an amplified RF signal. All communication with this antenna goes via FTP patch cable. At the end of this blog you find a more extensive discussion about this cable.

The movie below shows both the synchronized waterfall plots on the 40m band. I’ve tried to align the frequencies, the top window is the websdr and the bottom windows is from SDRsharp. Both receivers are some 120km apart and they might be picking up different signals, but as far as sensitivity and noise level are concerned the spectra are very similar. Some signals are weaker or stronger on one spectrum compared to another, the color coding is different, propagation is different, but most of it is the same.

So what do we have in the garden? The first part of the project was to build the antenna from thick copper wire and 40 mm PVC tube, the support mast is a 2.7 meter impregnated wooden pole that you get from the local garden center. The proper location of the active antenna is some quiet point, I checked this with a portable SW receiver. This point is roughly 12 meter behind our house.

Figure 5: Double loop antenna above the hedge in our garden, this was the first design, I don’t think it was bird and football resistent, so I replaced it.

I need 60m of FTP cable to reach the shack, at the other end of the FTP cable there is a controller, it needs 13.8 Volt, there are three switches and a Hammond box. The controller PCB which comes with the amplifier kit, it converts the HF signal into a 50Ohm coax connector, and this is what I provide to the airspy / spyverter combination.

Why this set-up? The main reason is that a small antenna at some distance away from the house will be less sensitive to local man-made noise, QRM, which comes from almost anything electronic within the house. It is the QRM level that typically adds 2 to 3 points on the S-meter scale on the 40m band. Turn the pre-amplifier on (from IPO to AMP1 on the Yaesu FT-991) and you increase the meter indication on the S-scale, but the signal to noise ratio does not improve, oftentimes the SNR simply deteriorates.

The worst offender is some nearby QRM source that is able to increase the noise level from S3 to 10db over S9, that is a whopping 46 dB! That same QRM source only adds two S-points (12 dB) in the back of the garden, and this means that you can still listen to the 40m band even when there is a lot of QRM. Usually the nearby high QRM  is on for 30min in the morning and up to an hour in the evening, it could be some electronic component about to die in a TV-set, it could be something in the kitchen, but I dislike asking my neighbors to locate what it is.

Figure 6: free space loss (vertically in dB) against wavelength (m) horizontally

Figure 6 shows what you can expect as noise reduction for an HF antenna that is 20m away from a source (the justification is that the shack is 20m away from the antenna, it follows from the free space loss equation that should be applied at a distance greater than the wavelength which is on the horizontal axis in meter. The dots on the graph correspond to various radio amateur bands. A QRM reduction of 20db or more suggested by figure 6 seems possible on the 14 MHz band, and this is very significant, however, the price to pay is a smaller antenna and probably less sensitivity, also, you need to get the signal from a remote antenna in the shack. The LNA will add at little as possible noise to the signal, but it will counteract against the damping of the CAT FTP cable.

Figure 7: comparison of dBFS and S-meter on the 40m band, horizontal S-meter scale where I allow 1.6 S-unit per 10 dB above S9. This was measure with the first antenna model

To verify the antenna sensitivity relative to what I already have (10 meter of wire end fed at at the roof of the house) I show in figure 7 the signal to noise ratio expressed in dBFS (dBs full scale, it is obtained from SDRsharp) compared to the S-meter scale of the Yaesu FT-991. The S-meter is supposed to be an absolute measurement, one S-point is 6dB and S1 is equal to -121 dBm. Around 7100 kHz I found that dBFS = 5.64 + 3.07 * S-meter-unit, the circles in figure 7 are measurements. The reality is that I can comfortably listen to a QSOs with a dBFS of as little as 5dBFS when I work with SDRsharp, whereas it is often a challenge to listen  QSOs on the FT-991 below S4 (when AMP1 is on on 20m, or when IPO is on at 40m). The DNR and DNF features of the FT-991 transceiver (see further down this blog where I discuss how to receive weak signals on the FT-991) certainly help to improve reception, however, DSP processing comes with certain limits.

The overall conclusion of this project is that an active-antenna is an improvement for my activities. What matters during reception is not as much the sensitivity of a receiver, but more the signal to noise level. A remote double loop antenna away from the local QRM has certainly improved my capabilities to listen to weak stations.

About UTP and FTP cables.

Probably the most challenging part of this project was to properly guide the CAT6 FTP cable under the garden, through the house and finally along the heater pipes to the shack. CAT 6 FTP cable is less expensive and easier to handle than quality coax cable, if you would design something with coax then also the power needs to go separately through the cable. It can be done with a T-bias connector, in that case you can not switch between both loops or a vertical which I can with the set-up from

FTP and UTP cables are transmission lines that come with four pairs of two wires each. The wire pairs are brown/brown-white, blue/blue-white, green/green-white and orange/orange-white. Inside FTP and UTP both wires in a pair are twisted so that the magnetic field caused by a forward current in e.g. the green wire cancels against a reverse current (of the same magnitude) in the green-white line. This technique is applied for all pairs, for UTP is it done without a mantle and with FTP there is a metal mantle.

For ethernet you use two pairs to send and receive, these are called TX+,TX-. Normally this is the green/green-white pair and for RX+/RX- one normally uses the orange/orange-white pair. The other cable strands are not used for ethernet. For the active antenna project you can use the colors the way you prefer on the RJ45 connectors, I recommend to make one or two debug cables (just 30 cm of FTP cable with open stripped strands on one side and a RJ45 connector on the other side). In this case also order one or more RJ45 CAT6 straight-through connectors because this allows you to check whether the strands in the RJ-45 are installed in the right order. Finally you also need pliers to crimp the RJ45 connectors. I had most of this material (debug cable, plier and connectors) already available because I installed CAT6 (and previously CAT5e) from the basement to the second floor.

CAT UTP or FTP cables with connectors is still sold because people apparently buy it. I refuse to buy cables with connectors because it is cheaper and easier to do this yourself after the cable is guided through all holes that you made in your floors, walls, ceilings etc. For most holes a single RJ45 connector would not fit or get stuck, but I can easily guide up to three CAT cables though that same hole.

Inside UTP cable there is no shield, thus no mantle, so UTP means unshielded twisted pair. UTP works fine for up to at least 100 Megabit per second to probably 100 meter. For higher data rates you need the foiled version of the twisted pair cable, so this is called FTP cable: Foiled Twisted Pair cable. The foil should be grounded, for this there is a separate 9th naked metal wire in the CAT6 FTP cable. Finally, UTP and FTP cables can be grey which is for in-house use, or it can be black when it is resistant to UV radiation for outside use. All cables that go underground should in my opinion be within PVC tubing, either flexible or normal. So for this project you also need something to guide the cable either through holes or pipes. When the cable is mounted against the mast in the garden it should be gently attached with tie wraps. The plastic mantle of the FTP cable should never be damaged, and the bending radius should in my opinion not be too small, but it may be shorter than with coaxial cable.

The CAT FTP cable is now 30 cm under the lawn and inside 16 mm PVC tubing, it goes under the kitchen, the living room and behind a curtain covers upstairs. In total I installed 60 meter of FTP cable, Murphey’s law caught me here, I ordered a roll of 50m FTP cable and had to order a second roll three days later to reach the radio shack. There are plenty of companies that ship you CAT FTP cable rolls probably within a day, it is less expensive per meter than quality coax cable. The damping factor of 60m CAT6 FTP cable turns out to be no problem for the active-antenna amplifier.

Also: you need CAT6 FTP cable with a solid copper core for this antenna project. RJ45 connectors are notoriously difficult to install. After trial and error I found that the following procedure works best, use the proper connectors that have a metal shield (the cable company will provide them, order more RJ45 connectors than you really need), insert the cable strands in the right order but do not strip each strand. (When you strip the cable strands it is very difficult, if not impossible to avoid short circuits in RJ45 connectors, so don’t strip the strands.) The plier will make the connections when you crimp the RJ45 connector, finally, check each wire in the RJ45 connector with both debug cables and the RJ45 straight through connectors.

Last update: 8-Nov-2016 12:36


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