UPDATE 21 Jan 16 – New high performance MF/LF converter completed. Details at the bottom of this page…Now completed…
UPDATE 28 NOV 15 – New page added ‘A RX Only Vertical’… new info!
Many of today’s transceivers, FT-897, IC-706, etc. etc. offer receive capability below the broadcast band – some good, others just OK. Often these rigs reduce their sensitivity below the 160m ham band. I use a FT-857 mobile rig for WSPR RX – it is driven by preamped loops and an E-Field probe.
A serious issue for many is interference from a strong AM BC station near ones QTH. Overload, images, hash, intermod and broadband noise can be common problems. Many of these problems can be cured by inserting a Low Pass Filter (LPF) between the receiver and antenna. Such a filter is easily constructed, and should be designed with the cutoff knee around 500 kHz.
Such a filter can be easily designed with free filter design software from ‘Almost Digital’, the same folks that make the L/C kit discussed on the VARIOMETER page. The link to the site is in HOT LINKS.
If you’re really getting crushed by a nearby AM station or aero beacon, a sharp bandpass filter might be the solution. I use a custom bandpass filter centered at 475 kHz. This filter is made by Jack Smith at Clifton Labs for about $45, complete with BNC connectors in a Rose box. Jack’s site is here: http://www.cliftonlaboratories.com/ . Jack, K8ZOA also makes a bunch of stuff for the MF experimenter, such as E-Field probe antennas and an assortment of preamps for loops. His site should be checked out, his gear is in kit form or ready built, top notch.
UPCONVERTERS FOR MF RECEIVE
A second option for MF receive is an upconverter driving your existing HF rig. Here, a mixer converts the low frequency signal up to an IF frequency such as 4.0 MHz or 10 MHz. Such a converter can easily be cooked up, but why bother when you can spend $14 for a kit converter with LO crystals for four AND 10 MHz ?? This rig is from Jackson Harbor Press, here:
http://wb9kzy.com/lfconv.htm About the cost of a good six-pack of Vermont beer. It is seen assembled above. You just can’t go wrong with this unit! Highly recommended. Hit ‘Ham Radio Kits’ from the link above…
This kit employs a 5-pole Chebyshev filter, designed by Clifton Labs. It will receive from 0-500 kHz – I easily copy WWVB at 60 kHz daytime with this converter…
A big advantage when using an upconverter is that your HF rigs functions, filters, notch filters, etc. can be used as needed.
For those with SDR capability above the AM BC band, a LF SDR receiver might be the answer. I use such a receiver with HDSDR software and it works well. It’s a Softrock LF receiver by Tony Parks (5-DASH), here: http://fivedash.com/ .
RX ANTENNA THOUGHTS
Your basic long random wire or 80M dipole is going to pick up MF signals, but don’t really make a good MF antenna. There are many better options –too many to cover in detail here, probes, flags, phased arrays, verticals, loops, pennants, on it goes.
I have specific experience with shielded, preamped loops and the so-called E-Field probe.
Loops can take many forms – I build either circular or square loops. One design is the ‘shielded magnetic loop’, Wellbrook makes ’em commercially, as does Pixel. Pricey, to say the least.
This is typically a broadband loop. A length of coax is prepared by cutting the shield in the exact center, leaving a 1/4″ gap, exposing the inner dielectric. The coax is mounted to a suitable, non-magnetic frame of wood or PVC. The ends of the center conductor drive a preamp with balanced front-end, and the two shield ends are grounded at the preamp. This so-called ‘magnetic’ loop functions as follows: The incoming signal’s H (magnetic) component induces a voltage in both halfs of the split shield, and thus to the inner continuous center conductor. A signal received broadside to the loop is cancelled to some extent, and signals received in the long plane are enhanced, thus the loops exhibits directionality. If the loop is carefully constructed and symmetrical on both sides of the coax shield cut, significant signal rejection occurs broadside. This type of loop consists of a single turn only of coax.
The five foot loop was monunted on my shed with a rotator.
SHIELDED LOOP PREAMP
The magnetic loop requires a preamp with balanced input. A center tapped micro audio transformer was used, the active device is a J-107 FET.
A custom PCB for the preamps was designed using ExpressPCB tools.
TUNED RECEIVE LOOP
A tuned loop is exactly as stated. A multi-turn loop is resonated with a variable capacitor at the frequency of choice, and the signal is picked up by a one or two turn ‘secondary’ winding near the main loop and preamped. The tuned loop typically produces a stronger signal than the single-turn broadband loop described above. However, if remotely located, provision must be made to tune the antenna. To accomplish this, I have built electronic remote tuners using the VARICAPs found in auto radios. Tuning is accomplished with a pot in the shack.
Searching the web just now, I found over 20 GOOGLE pages describing loop antenna projects of all flavors.
The preamp for this loop is designed by Larry W7IUV on his highly informative website, here:
w7iuv.com Lot’s of MF/LF stuff here. Larry is also WH2XGP Part 5.
E-FIELD PROBE ANTENNA
The final antenna I’ll discuss is the so-called E-Field probe. This is a fairly simple antenna – essentially a short whip or rod, under 10′ in length, connected directly to a high-gain preamp.
As it’s name implies, it responds to the E (electric) component of a signal. The very short whip, exhibiting very high impedance, is directly coupled to the gate of a high impedance FET. Standard gain stages follow. This antenna works very well when correctly positioned away from domestic noise sources, as it is subject to noise. Mine is placed in the woods 100′ away. Jack at Clifton Labs has a detailed explanation regarding placement and coax common-mode noise reduction on his site, referenced above.
High Performance MF/LF Upconverter
The first winter project resulted from the desire to create a ‘better’ upconverter for the low bands. This converter is optimized for the bands from essentially 1 kHz-600 kHz.
The front-end consists of a 5-pole LPF directly driving a Minicircuits SBL-1 mixer with no active preamp. Originally I was going to use a high performance mixer, purchased on Ebay, that required +22 dBm LO drive. This would have resulted in a high IP3 converter, but being in a location free of AM broadcast stations and the possibility of intermodulation problems, I went with the SBL-1 which requires only +7 dBm of local oscillator drive.
Each section of the LPF, the mixer itself, the LO brick, and the output circuit are isolated by copper shields soldered to the copper (PCB) groundplane. The LO was constructed on a piece of perfboard prior to being mounted in its box. The oscillator is at 5 MHz – an oven temperature stabalized brick with electronic frequency ‘tweak’. Any LO brick or el-cheapo stamp sized oscillator will work – either sine or square wave output is OK. The 3 volt output of the brick is buffered by TTL drivers and delivered to the SBL mixer via a 50 ohm pad to insure that the mixer sees a 50 ohm load and +7 dBm drive (~ 1.4v p-p into 50 ohms).
The LPF rolls off at 600 kHz as I wanted flat response at the critical 475 kHz region. A -2 dB pad between the filter and mixer insures a 50 ohm presentation to the latter.
In order to insure that the output of the mixer sees a 50 ohm load, a microwave amplifier chip (MMIC) is used as the post amplifier – they present a constant 50 ohm load across a very wide range of frequencies.
It should be noted that the IF and RF ports of the mixer are reversed – the LPF drives the IF port, and the RF port drives the post amp … the reason being is that the ‘standard’ usage of the RF port as input is limited to 500 kHz and up for the SBL-1. The IF port is rated from DC and up…hence becomes the input port.
Initial tests look good. Daytime reception of WWVB at 60 kHz resulted in a booming signal, and with no antenna there is absolutely no additional converter noise in the IF receiver – here, a FT-857d. All the usual LF beacons were heard including weak ones not previously in the log. Now, to box it up and finish the project…
More to come as available!
CU on the band.