Receiver System Sensitivity tests on LF and VLF
Ever since I became interested in 73 kHz and 136 kHz I have been haunted by the question "Is my receive system as good as it should be". This was reinforced by the occasional sound of stations transmitting CQs over continental DX., which I could hear being close to the East Coast. I have a number of receivers that cover or at least claim to cover the frequencies of interest. My object was to determine the weakest signal I could reliably detect and then to attempt to relate this to a definitive measure of field strength. The ultimate aim was to estimate whether it would be possible to hear a signal from the AMRAD beacon stations WA2XTZ. My interest was further stimulated by the monitoring work of Dick Rollema PA0SE and Vaino Lehtoranta OH2LX , which was discussed on the RSGB LF Internet reflector.
Equipment
The most obvious and most important piece of equipment required was a signal generator. I found that whilst I had a series of reliable signal sources from 400kHz to 11Ghz, I had nothing reliable in the LF range. My first experiments were with audio and video generators, which are readliy available and cover ranges up to 1MHz. These units proved very useful for tuning aerials and measuring inductors, but were totally useless for measuring receiver sensitivity. The reason is quite obvious. Audio and Video generators are designed to operate in the 1mV and upwards output range. The case and attenuator leakage levels mean that an S9+30dB signal can be indicated on the receiver without the generator connected. A quick check on this is to disconnect the coax lead to the receiver and just touch the connector housings together. You will often get as big a signal indicated on the receiver S-meter as with the coax connected properly. In fact I use the 2nd harmonic case leakage of a Farnell DSG1 sythesised generator to provide a calibration signal on my FFT waterfall display.
A quick look inside a good quality RF generator will show why this happens. A Marconi TF2002, 10kHz to 72MHz generator has triple screening of the RF circuits, good quality coax interconnecting units inside the generator, including a coax-screen choke between the generator section and the attenuator. These generators are now getting a little long in the tooth (manufacture early 1960s) and have germanium transistors which are not particulary reliable. I have 2 of these units (non-working) but was fortunate to aquire one of the later TF2002B units. These have basically the same circuitry but are built with silicon planar transistors in the 1970s. The instrument is very stable when warmed up and has fine frequency adjustment and the capability to externally modulate the the signal in amplitude or frequency from an analogue source. The lowest calibrated output from this generator is 100nV into 50ohms, but the case leakage levels are so good that sensible measurements below this level can be made with external attenuators.
Receivers
I have always regarded receivers as valuable test equipment, and now regret parting with my early valve receivers. I decided to apply my measurements to a Lowe HF150, an AOR7030+, a Palomar LF converter with a Trio R1000, and an IC-706G. I will not consider the problems of intermodulation distortion products, as these are well covered in many texts. It should be remembered that with the close proximity of strong commercial stations in the LF region of the spectrum, good performance in this quarter is more important than good sensitivity.
I have assumed that all aerials will be matched to 50ohms and input to the rx will be through the 50ohm coaxial input. There is a problem with this assumption. It is possible at these low frequencies that the input impedance of the coaxial input may not not be accurately 50+0j ohms.
Measurements
The first stage is to calibrate the S-meter (where such is included). I have standardised on measuring the level for S9, S5 and S1, but if you require to derive field strength measurements you really need to calibrate every S-point in dBs
The next measurement is often refered to as the minimum detectable signal. It is difficuly to define this in a really repeatable way. I have considered that the receiver is to be used on LF for morse so I considered the levels at which I think I could decode a fairly deliberate morse transmission, I also consider the level at which I can reliably determine the presence of a carrier, even where it is too weak to read the morse, is important. The question is under what IF bandwidth do you make these rather subjective measurements. I find a 2.5kHz SSB bandwidth a useful state as it is available on most receivers. Readability can be enhanced by the use of narrow filters, but im my experience, confirmed by others, not as much as theory would lead us to believe. The concensus is that there is always some increased discrimination due to the capablity of the operators brain to extract a tone from the noise. Finally not really a measurement of the receiver, but at what level can I detect a carrier using an FFT waterfall display. All these measurements are carried out with the signal source connected directly to the rx. The next stage is to determine the same parameters when receiving a signal from the aerial, with all the attendent noise.
Injecting a weak signal
Whilst the ultimate sensitivity of the receiver is an interesting measure, a more useful parameter is the level of signal you can receive in the band noise. What we need to do is to generate a weak signal from the signal generator into the receiving aerial.. I have achieved this by feeding the output of the generator into a 9turn 20cm diameter loop of thick wire( 10 amp auto wire we used for wiring land mobile power connections) fed from the generator by a few metres of RG58 coax. This will of course not present the required 50 ohm load to the signal generator, so the 1dB steps attenuator will not read accurately. Because at this stage we are not interested in the absolute level we can connect an attenuator pad at the end of the coax feeding the injection loop. This ensure the generator see the correct termination and the attenuator works correctly. The signal generator is moved about 5metres away because even a good generator like the TF2002 does have some case leakage.
I use a 16 turn 1.2metre diameter tuned loop for reception, so the injection loop was set up coaxially with the receive loop and at a distance of at least twice the diameter of the receive loop. The signal generator is wound up to give an S-meter reading whose level at the input terminals is known from the calibration (say S7 or S8 ). Let us assume this level is 25uV. We now forget the voltage readings and just wind in dBs on the signal generator until we can just hear the signal. If we have added say 60dB of attenuation then out injected signal is presenting 25nV (25uV -60dB)at the receiver input. I have not included any measurements as I considered them to be more relevant to my aerial than the receivers. In general under good conditions, low local noise and no static crashes, the minimum discernible signal approaches a level 6dB worse that directly injected into the receiver. This may well indicate that I do not have a big enough aerial!
According to the "Radio Engineers Handbook" by Terman and also "Radio Engineering" by the same author it should be possible to calculate the field strength of the injected signal. The trouble I have is that two different equations are given and there does seem to be some dimensional errors in one. Also I believe the injection loop needs to be the same diameter as the aerial loop, which I have not tried yet.
Field strength measurement
It is possible to determine the field strength at your location by constructing a 'standard aerial'. This is not as complex as it sounds as often a single turn loop will suffice. The terminal voltage of the loop for a given field strength is given by the equation:-
e = 2*pi*E*A/l where A = area of loop in metres squared
E = field strength in Volts/metre
l = wavelength in metres
e = terminal voltage of the loop in volts
Because the impedance of a single turn loop is low with respect to the 50ohm input of the receiver one may read off the loop terminal voltage from the calibrated S-meter.
Suppose you use a reliable commercial signal like DCF39 in Europe (located at Burg near Magdeburg, and with a carrier frequency, or more accurately 'mark' frequency, of 139.83kHz). Orientate the loop to achieve the maximum signal strength, and read the S-meter. Calculate the field strength from the above equation. Then connect your normal aerial, read the S-meter for DCF39. This reading now represents the previously calculated field strength. You can read of the field strength of another station by relating the difference in dBs as measured on your S-meter. This is an absolute measurement, and much more meaningful than a undefined S report. It has the added advantage that it helps you to ensure that your equipment is still working correctly. BUT TAKE CARE!! More than one LF enthusiast has started to rip his station apart of the rare occasions when the Burg signal is off-air.
If the S-meter in the receiver has a very non-linear scale you may wish to carry out the measurement by using an external attenuator. The reference signal, and the signal to be measured are both reduced to the same S-meter reading (say S2 or S3) by adjusting the external attenuator and the difference in attenuator settings used to calculate the the field strength. You can see from this why receivers with accurate dB scale strength meters are sought after and why several stations use now-redundant laboratory-grade Selective Level Measuring sets as receivers at LF.
Conclusions.
I was surprised to find that the performace of the HF150 was so good. It follows that I had a reasonable success during my first few months of listening on 136kHz with it. I suspect it is intended for broadcast listening and has only a 1kHz resolution on the display. Despite this tuning is quite easy and I was able to set the span on a DSP waterfall display to better than 10Hz. It is small and capable of operating from internal batteries, so it might make a useful small receiver for holiday listening from exotic locations.It does not have a narrow (CW) filter nor is there any way of disabling the AGC. The latter is most important on LF bands where static crashes disable the receiver for the period of the AGC hang time. I have found its intermodulation performance to be adequate and though its filter slope is not as steep as some it is able to keep DCF39 on 138.83 some 60dB down. There is a trace of the 'carrier' on a DSP waterfall display under quiet conditions on the image frequency.
The AOR 7030 was purchased for its supposed strong signal performance, and its capability to fit narrow filters. I have not been disappointed with the performance. It has DDS local oscillators which are very clean. If anything the AOR falls down for a serious operator on its ergonomics. It has been squeezed into a small box with a minimum number of multifunction knobs and buttons. The functions of these controls is accessed by navigating a menu system. This unfortunately means a sequence of up to six button presses to put the AGC off, and the same to put it on again. The S-meter does not indicate with the AGC off. The unit can be fitted with an optional 500 Hz Rockwell-Collins filter available from AOR, or a similar physical sized 300Hz filter available direct from Rockell Collins (Part No.
The Palomar LF Converter is of unknown age. It does not have any model or serial number, and from my memory of old Ham Radio (US version) was probably produced as far back as the 1980s. I found its performance very disappointing even with a sensitive receiver. I think the basic sensitivity of the R1000 on the IF (3.5MHz) is about -20dBu (0.1uV) so there would seem to be conversion loss. I would recommend one of the designs produced by some of the other LF operators and detailed on their web sites.
The Icom IC 706MkII is specified as operating between 160m and 2m, so I suppose that although it tunes 136kHz, the frequency is really outside its main range, and its poor sensitivity is to be expected.. I believe it is used as a drive source for high power FET amplifiers. Its receive performance might be adequate with suitable preamplification, but the synthesiser noise was most noticable in my tests. I suspect that the noisy local oscillator may lead to less than optimum receive performance even with adequate preamplification.
Measurements made are recorded below