Low Noise Antennas and Receiver Front-End Protection
Vince Lear G3TKN discusses the siting of receive antennas and protection of the receiver front end.
Vince Lear G3TKN discusses the siting of receive antennas and protection of the receiver front end.
The vast majority of radio amateurs use the same antenna for both transmit and receive. On the HF, VHF and UHF bands, the transmit antenna will generally offer the best performance on receive; especially if it is directional.
However, when we get to the lower frequency bands such as 160m and 80m, results can change considerably. This is especially true for those who live in suburbia who are in close proximity to other houses.
Most households these days contain a wide range of electronic devices and many of these devices are responsible for emitting noise across the spectrum. It is, of course, a cause for concern for all of us and these issues have been well documented in the amateur radio press.
Signal-to-Noise Ratio
A limiting factor for clear reception on all bands, but particularly on the lower frequency bands, is the signal-to-noise (s/n) ratio. If we have a good transmit antenna, we can expect it to receive well, in terms of signal strength. However, it is of little consolation when we are receiving a station at 59+10dB if our local noise level is 59+5dB! Yes, we will still hear the station, but it will be uncomfortable listening with the background noise only 5dB less than the wanted signal. If there is a station below this noise level, then we are not going to be able to copy them at all.
We therefore need a receive antenna that offers us a much improved s/n compared with the antenna used for transmission.
It is completely superfluous what our S-meter reads when we are using a low noise receive antenna; the essential factor is that we can actually hear the station more clearly even if the S-meter reads quite low. Hence, when assessing the effectiveness of an antenna designed for receive only, it is important to concentrate on what we are actually hearing and disregard the S-meter reading.
To look at a practical example; I run a top fed, trapped, half sloper for 80m and 40m off my tower at 50ft. The tower supports a Hygain TH5DXX triband beam, which effectively acts as the other half of the half-sloper. It is an effective low angle radiator on transmit, but a noisy receive antenna on 80m. In the middle of the day the S-meter normally reads S9+5dB on background noise. However, using my
Wellbrook loop (placed in the middle of my neighbour’s garden) the background noise is considerably reduced such that the S-meter only reads S4. The really important point here is that stations sound far more pleasant to listen to on the Wellbrook than on the half-sloper. This is not always true at night with all stations; sometimes the half-sloper does a good job of receiving DX while at other times the Wellbrook Loop is better to receive on.
However, on 40m, there does not appear to be the same problem and in the middle of the day on a clear frequency the S-meter only reads S3-4 on the sloper, so there is little need to switch in the Wellbrook. Sometimes at night some stations might be better copy on the Wellbrook while at other times the sloper might provide clear reception.
Always be prepared to switch between the main transmit antenna and the lownoise receive antenna to see which one gives the best readability at any one time.
On 160m, I sometimes run a quarterwave inverted-L with a 50ft vertical section. This is a very noisy antenna on receive but a good DX antenna on transmit. I do find that on direct comparisons between it and the Wellbrook loop, the latter is nearly always the better antenna in terms of s/n ratio.
Some years ago I ran a 160m half wave inverted-V- dipole with its apex at 70ft. I
compared this against a homemade 2m diameter passive tuned loop on 160m. I RL1 RL3 found that most of the time the inverted-V dipole compRarLe1d very well on reDc1eptionDo2f Stateside stations to the passive loop. With 1 N 40021 N 4002 some ext re mE en lyerwgi es ian kg c so tail st ions the loop appeared to have a slight advantage at certain times but it was very marginal.
Despite the relatively low height of the inverted-V in terms of wavelength on 160m, it did work reasonably well for DX during ‘greyline’ conditions. However, as expected, I have always found a vertical (low angle) radiator to be a more consistent performer when it comes to working DX on 160m.
I mention this practical example to highlight that you always need to experiment when it comes to receive antennas and there are no hard and fast rules!
Experimentation
In the February 2021 edition of PW, Joe Chester M1MWD wrote about low noise antennas in his article Low Noise Antenna Options for Small Gardens. There are plenty of designs listed in his article that you can experiment with.
My advice is to experiment with different types of low-noise antennas until you find the+1o2nVe that gives you the best results.
Although I was initially going to write about receiver protection when using receive antennas, I felt it would not be amiss to also write about my own personal experiences when using separate antennas.
It is important to remember that what might work well in one location might not be so effective in another location; hence ‘experiment’ is the key word when considering low-noise receive antennas. As I said earlier, there might well be times when the transmit antenna offers better performance on receive and it is not necessary to switch in a separate receive antenna.
I would just like to add some of my own observations on receive antennas before I discuss receiver protection.
The Low Dipole or RandomWire
On the low bands, where it is not possible to place horizontal antennas sufficiently high enough to obtain low-angle radiation, vertical antennas tend to be the antenna of choice when it comes to working DX due to their low angle of radiation. However, when used in suburbia, they tend to pick
Fig. 1: The protection diodes on Wellbrook loop, which the author resoldered and placed in heat shrink tubing for weather protection.
Fig. 2: The homemade unit showing the inside of the plastic box and assembly. Fig. 3: Circuit of a simple homemade protection device using a pair of reed relays and a single DPDT relay. This had a ‘failsafe’ system built in such that if the +12V DC supply should fail, the first reed relay would isolate the receiver antenna input.
Fig. 4: The KD9SV Rx protector built from a kit and again showing assembly inside box.
Fig. 5: Circuit diagram of the KD9SV protector. Fig. 6: The DX Engineering DX5000HD Receive Guard.
up a lot of noise on receive. I have known of some low band DXers who have used simple low horizontal dipoles (6-12ft high) wrapped around the garden (or using a kind neighbour’s garden if allowed!) to obtain an improved s/n ratio on receive. Sometimes just a random wire can be effective when run along a fence line.
Placement of the Receive Antenna
I have always found that it pays to place any receive antenna as far away from your own house and other people’s houses as
possible. Once again, it is always worthwhile to experiment with the placement of a receive antenna to find the optimum position where it will pick up minimum noise.
If you are using a small loop antenna, such as the Wellbrook loop, it is advantageous to be able to rotate it. However, I do not have a rotator on mine and have just set it such that it picks up minimum noise. On 160m, I find that my most annoying noise is from some power lines not too far away. Fortunately, this noise can be completely nulled out by suitable rotation of the Wellbrook loop.
Noise Pick Up on Coax feeding a Receive Antenna
An important point with any low-noise receive antenna is to make sure there is a good RF choke in the coaxial feed to prevent noise pick-up on the outer of the coaxial cable feeding the antenna. This is, of course, important on transmit antennas too where current flow on the outside of the coax may result in EMC issues on transmit.
There are plenty of designs for such chokes on the excellent site of the late Steve Webb G3TXQ:
Noise Pick Up off theTransmit Antenna
Another factor that is often overlooked when considering low-noise antennas, is their tendency to pick up noise from the main transmit antenna; especially if they are in fairly close proximity to it. Ground plane antennas with elevated radials can often be a cause for concern in this respect.
The late John Devoldere ON4UN, in his excellent book Low Band DXing mentions this specific point and discusses de-tuning of the transmit antenna. Fortunately, I have never found this an issue when using my Wellbrook loop although it is placed in my neighbour’s garden and not over the radial field that covers my own garden! This may help but I really can’t be sure.
Transceiver with no Separate Receive Antenna Input
If you have a transceiver that does not have a separate receive antenna socket, DX Engineering offer the RTR-2 Modular Receive-Transmit Interface. This will allow the standard transceiver owner to enjoy the advantages of separate receive antennas. These interfaces also add inexpensive failsafe protection for the unswitched receive antenna (RX ANT) port.
These devices are very useful for transceivers such as the Icom IC-7300 and other models that do not have a separate receive antenna input.
Receiver Protection
How much protection manufacturers give to the receiver front end when a separate receive antenna is used seems to be somewhat of a grey area! I have found little information on this topic in transceiver specifications and I think it is something that deserves more investigation. I have spoken to one dealer recently who told me that they have indeed had transceivers sent back for repair after receiving front end damage, due to being connected to a receive antenna without proper protection. The amount of signal received on the receive antenna will depend on the distance it is from the transmit antenna, as well as the power being transmitted.
Low-noise receive antennas can be divided into two main types; ‘passive’ and ‘active’. In the passive type, (e.g. EWE, K9AY, Pennant, Beverage) no amplifier is used and therefore the protection must be placed at the transceiver receive antenna input. In the active type (such as the Wellbrook loop), it is the amplifier built into the loop itself that must be protected. Under very strong signal conditions, it is this amplifier at the loop that will saturate and prevent damaging signal levels going to the receiver input.
However, under these conditions it is quite possible for the loop’s own amplifier to be damaged. Fortunately, in the later models of the Wellbrook loop, the amplifier can be replaced, although, of course, this comes at a cost.
Wellbrook will incorporate protection diodes if required across the input to the amplifier. In my own Wellbrook loop, there are a total of four diodes; two in series in forward bias and two in series in reverse bias. These combinations are placed in parallel so as to have a breakdown voltage of 2 x 0.7V = 1.4V. If only two (back-to-back) diodes had been used, the breakdown voltage would only have been 0.7V, which could cause rectification of strong signals at the amplifier.
I have placed the protection diodes in my own Wellbrook loop in heatshrink tubing for weather protection as can be seen in Fig. 1.
Failsafe Protection Device
I built this very simple protection device shown in Fig. 2 some time ago. The circuit is shown in Fig. 3. This device will isolate the receiver antenna (via reed relay RL1) if for some reason the +12V DC supply to the
protection unit should fail because RL1 is permanently connected to the +12V rail.
When the junction of the two diodes (IN4002) is taken to ‘ground’ via the transceiver’s send/receive port, both the IN4002 diodes are forward biased and the linear is switched in. In addition, the second reed relay (RL2) is activated so grounding the receiver antenna input. The reed relays operate very quickly so the receive input will be earthed before the linear amplifier operates. In addition, the DPDT relay (RS Components RS12) is switched so that both the inner and outer of the coax from the receiver antenna are isolated. A plastic box must be used for mounting the antenna socket so that the outer of the socket (phono or BNC) is isolated from ground.
I used this unit when I operated as ZL1VL in New Zealand (2006-2013). I ran a Hustler 6BTV vertical with a 130ft Beverage, which I sometimes used on 20, 30 and 40m. As I lived in suburbia, the Beverage often provided a better s/n ratio than the Hustler vertical. I also ran an Alpha 99 linear amplifier at 1kW (which was our legal maximum in NZ) so the Beverage (the feed end was only some 10ft from the vertical) must have picked up quite a hefty amount of RF! Despite the simplicity of the circuit, I had no issues with the front end on my Icom IC756Pro2, so the unit obviously worked!
KD9SV Protection Device
The KD9SV Protection Device is more sophisticated and can be seen in Fig. 4 with its circuit at Fig. 5. It was supplied as a kit and was built by my friend Rob Allbright
G3RCE. The latest KD9SV protection devices are now supplied by DX Engineering and are fully assembled: www.dxengineering.com
The kit version appears to be no longer available although I am not certain of this.
DX Engineering RG5000HD Receiver Guard
I recently bought a DX Engineering RG5000HD Receiver Guard, Fig. 6, to use with experimental passive low noise receive antennas. The main advantage with this unit is that it is passive so does not require any DC supply for it to operate. The unit simply connects in series (using BNC connectors) between the receiver antenna input and the receive antenna. No other cables are required as used in the previous devices mentioned.
Its principle of operation relies on the saturation of a ferrite transformer under high RF signal levels. Its specification appears in the sidebar. Further details are provided on DX Engineering’s website (see earlier) and it can be obtained in the UK from Martin Lynch & Sons. A very comprehensive 12-page booklet giving plenty of technical details of the unit is supplied with it.
Although more expensive than the other protection devices, it has the advantage that it only has to be connected in line with the receive antenna.
It is worth remembering that a burntout receiver front-end is likely to cost a lot more to repair than the cost of the RG5000HD!
Beware though, the RG5000HD must only be connected in the receive antenna. It will be damaged if it is connected to the transmit output. The information booklet that comes with it makes this very clear.
Conclusion
Before experimenting with any passive low-noise receive antennas, it is important to make sure the receiver front end is fully protected against high RF voltages that may be induced into the receive antenna due to its proximity to the transmit antenna. This is particularly important if high power is being used and there is limited separation between receive and transmit antennas.
If you are using an active low-noise receive antenna, such as a loop or short monopole with active amplifier at its base, then it is this amplifier that needs the protection.
Once adequate protection is built into the system, then you can happily experiment with different receive antennas in the knowledge that the front end of the receiver is fully protected.