Amateur Radio – Engineering Radio

The Ultimate Receive Antenna

This post is about a Large Loop On Ground (LLOG) receive antenna.

I have, for some time, been lamenting the demise of my K9AY antenna last year. While I don’t spend a lot of time Shortwave Listening these days, it is sometimes a nice diversion on a rainy day.

I have been reading about LOG (Loop on Ground) broadband receive antennas for a few years and the concept sounds interesting. Loop antennas in general, tend to reject RF noise better than dipole or vertical ground plane type receivers.

The idea is the loop antennas are more H field (magnetic) than E field (electrical) receivers. Most unintentional RF noise generators create noise in the E field.

Near Field vs. Far Field Electro-Magnetic energy

Additionally, close proximity to the ground further lowers E field noise due to capacitive coupling with the ground. The ground acting as a sink for the E field noise. Loops also tend to have better rejection of chaotic Near Field energy. Thus the combination of those three things will create slightly lower signal with much better signal to noise ratios over traditional up in the air antennas.

The downside is that it cannot be used for transmitting. The capacitive coupling noted above will quickly dissipate all of the transmitted RF into ground as heat.

The LOG antenna that most amateur radio operators use is either or close to the antenna outlined by Matt, KK4JY: The Loop on Ground Antenna

That is all well and good, but I want something larger to go down through the entire Medium Frequency band in into Low Frequency range. The main reason is I want to start fooling around with the 2200 and 630 meter amateur radio bands. Also, wanting a good low noise AM broadcast receive antenna. With that in mind, I began taking inventory of available real estate and parts.

As for real estate; I own a patch of woods behind my house that is approximately 200 x 200 feet. Good enough for a fairly large square loop. The good thing about particular plot of land is it is far away from the neighbors, the utility lines and other sources of electrical noise.

As for the available parts; it turns out I had almost everything needed. The only thing I needed to purchase is a water tight outdoor enclosure for the matching transformer, from Amazon for $9.00.

Here is a list of items used:

Item	Amount	Use	Comment	Cost if new
Wire, 14 AWG	470 feet (143.26 M)	Wire Loop	Pieced together from left overs	$100.00 (500 ft spool) at Home Depot
Toroid, BN-73-202	1	Transformer core	Left over	$0.95 at Amidon
Wire, 26 AWG magnet wire	18 inches (46 cm)	Transformer winding	Left over	$9.00 (2 oz) at Amazon
Coax Cable, Cablewave FCC38-50J	175 feet (53.34 M)	Transmission line to house	Salvaged from decommissioned AM directional	$53.00 (500 ft spool RG-6) at Home Depot
RF Connector, type N Female	1	Antenna output	Found in junk drawer	$6.00 (6ea) F chassis mount at Amazon
Enclosure, ABS water tight IP67	3.9 x 3.9 x 3 inch (9.9 x 9.9 x 7.62 cm)	Transformer housing	Purchased from Amazon	$9.00 at Amazon

The wire is copper THHN 14 gauge. If buying new, this is the single largest expense. I soldered and heat shrunk several lengths from left overs on spools. This antenna is going to be in the shade, so I don’t have to worry too much about UV degradation of the insulation. If that happens, I can find some more insulated wire.

The big bonus is the Cablewave FCC38-50J phase stabilized cable, most of which is buried out to my old K9AY antenna, I simply extended it about 50 feet. Having the line already buried to much of the manual labor out of the project. The buried section of line acts as an RF choke, fully eliminating any electrical noise coming from my house at the antenna side of the line.

Large Loop On Ground antenna, installed in woods behind my house

I did have to go around and clear away some brush and dead trees along where the antenna wire was laid on the ground. I wanted to make sure that the wire was directly on or within an inch or two of the ground.

LLOG antenna transformer and transmission line

The transformer took about 20 minutes to construct. It consists of six windings on the antenna side and two windings on the transmission line side. I used a type 73 core because I am more interested in the medium frequency range. It seems to work well up to about 15 MHz. The transformer is for galvanic isolation, keeping the loop current balanced and separated from the transmission line. The shield is not grounded at the antenna side, but is grounded where it comes into the house. On the transformer enclosure, I used a salvaged N connector to match what I had for the transmission line. Any low loss cable, including RG-6 or RG-11 will work very well.

WKNY signal, 18 miles distant from transmitter

After a few hours of low intensity work, I had everything together. I connected the antenna to my trusty Kenwood R-2000 receiver and tuned to the nearest Class C AM on 1,490 KHz. Wow! What a difference! This little 1 KW high band station 18.6 miles (30 km) away barely came in before, especially when the ground is dried out. The relative signal strength meter on the receiver shows an S9+5. I am located on the edge of the predicted 0.5 mV/M contour while the spectrum analyzer shows a -65 dB signal, which is netting a 44 dB SNR, which is very good. All of this is after a Mini Circuits ZSC-4-2 power divider, which has an insertion loss of -6 dB per port.

Wide view of the entire AM band, about 1pm

One of the reasons for the “decline of AM;” overcrowding. This was taken during the daytime, when there was no skywave propagation. Barely an open frequency.

With the spectrum analyzer and made a few interesting discoveries. First, the noise floor in the Medium Frequency range goes from about -100 to -110 depending on the frequency. The noise floor on High Frequency is -112 or lower across the entire band. I clearly get all of the NYC AM stations as well as most of the stations in and around Albany. Before sunrise, I tuned around the 630 (472 to 479 KHz) meter band and heard a few stations sending CW (W4TS @ 164 miles (264 KM), K2ORS @ 157.2 miles (253 KM)). A few overnights of WSPR monitoring on 475.6 KHz netted 194 spots from 11 different stations. The furthest was 822 miles (1,322 KM) away in central Illinois. All of these stations are transmitting 5 watts EIRP or less.

This antenna works well to about 15 MHz, where the signals start to drop off because of the transformer material that I used.

Using the Network Analyzer, I found the resonance is 2.865 MHz with a -36 dB return loss. This makes sense, as the wire laying on the ground will have a lower velocity factor due to the capacitive coupling with the earth. In this case the VF of the antenna is 73% when the soil is dry. The velocity factor should go down when the ground gets wet, or covered with snow.

It might be interesting to do some ground conductivity measurements…

Designing filters for 630 meters

This has nothing to do with broadcasting. It does, however, have a good deal of geeky goodness.

I have started a new project, getting on the air on the 630 meter Amateur band. For those who do not know, 630 meters is from 472–479 kHz which is below the AM (or Standard) broadcast band. It was formerly part of the Maritime Mobile allocation. For US Amateurs, these frequencies were added in 2017 so it is a relatively new experience.

There is no commercially available equipment for this band, so it depends on the potential operator to make his or her own equipment which is where the fun begins.

To start, I thought I’d repurpose a WSPR beacon to 630 meters to do some antenna experimentation. Like all transmitters, the output of this unit needs to be filtered to reduce or eliminate out of band emissions. The Amateur radio service falls under Part 97, which has somewhat different requirements than Part 73 or 74.

47CFR 97.307(d) states:

For transmitters installed after January 1, 2003, the mean power of any spurious emission from a station transmitter or external RF power amplifier transmitting on a frequency below 30 MHz must be at least 43 dB below the mean power of the fundamental emission.

That is a fairly low bar. I am going to shoot for something better. WSPR beacons center around 475.6 kHz. The harmonics are at 951, 1428, 1902, 2378, 2853, 3329, 3804, 4280 and 4756 KHz. The first two are in the AM broadcast band. A quick look at the Zachtek WSPR beacon show these harmonics:

unfiltered ZackTek WSPR Desktop transmitter, 630 meter band

Definitely does not meet the out of band emissions standard set forth in FCC 97.307. Typical of solid state amplifiers, the odd harmonics are greater than the even.

I used a filter design program called Elsie to design suitable filters for 630 Meters. There are two types of filters that can attenuate the harmonics; low pass and band pass. A low pass filter passes all emissions below the cutoff.

That is fine, however, it does not eliminate the possibility of interference and inter- modulation from frequencies below the band. Both types of filters are also good for receivers in the presence of AM broadcast band towers, which can desensitize receiver front ends when located nearby.

A band pass filter cuts off frequencies above and below the pass band.

This is a nodal inductor-coupled band pass filter. I like this design because it has deep shoulders and has better performance with the harmonics in the AM broadcast band.

Prototype low pass filter:

Quick prototype board. Capacitors are Cornell Dubilier silver mica dipped, the inductors are wound on T130-3 material. The SWR and return loss:

Smith chart:

The Smith chart shows that it is slightly inductive on the desired frequency. The way to mitigate is to either add some capacitance (not easy) or reduce the inductance (somewhat easier). I tried tuning it by changing the spacing on the windings of L1 and L2. There was no change.

Low pass filter response:

The second harmonic on 951 KHz is -58.37 dBc. Harmonics 3 – 7 are 40 dB below the fundamental. This is adequate but the band pass filter below is better.

Prototype band pass filter:

The above filter is a little rough, but it was a good test of the filter program’s design parameters. The capacitors are Cornell Dubilier 0.02 uF 500V. The inductors are wound on T130-3 iron powder toroid cores. The results are good:

By adjusting the spacing of the windings on L3 (center of the board), I can tune the VSWR and Return loss for best values.

The same ZachTek WSPR transmitter noted above, running through the prototype filter:

ZachTek 630 Meter band pass filter response

The second harmonic on 951 KHz is -62.86 dBc. The rest of the harmonics are less than that.

Of the two, the band pass filter has better performance characteristics. The return loss/SWR is lower and can be tuned by adjusting the spacing of the toroid windings on L3.

I decided to take the next step and make a PCB. I have KiCad on my Linux machine, which works well. Sometimes some of the foot prints need to be edited so the dimensions are correct, but that is easy.

What I like about this board design is that it will work on any amateur band below 7 MHz with different component values. I had five boards fabricated and built one of them out. The T130-3 toroids are wound with 14 AWG magnet wire. The capacitors are same used in the prototype, 0.02 uF, 500V mica dipped.

Board mounted in aluminum enclosure:

Band pass filter mounted in diecast aluminum enclosure

This build has similar measurements to the prototype board above. Based on what I found while making this, I made a few tweaks to the circuit board in KiCad. I would consider selling these, if there is enough interest.

Weak Signal Propagation Reporter

Slightly off-topic, but includes radio.

The antennas are the most interesting aspect of Radio Frequency Engineering to me. The transfer of power in the form of voltage and current to the magnetosphere and back again is where the rubber meets the road. Any opportunity to experiment with the art of antenna design and fabrication is welcome.

This is for the Amateur Radio community. With the upswing of Solar Cycle 25, predicted to peak in July of 2025, I decided it would be fun to get back on the air with some type of HF setup.

My past experience with HF radio and peak solar cycles is that wild fluctuations can occur creating band openings at unusually high frequencies or no propagation at all. The geek in me finds this very interesting. HF Propagation is a complex matter. Long-distance communication can be carried out with very low power levels provided the ionosphere is bouncing signals back to the earth instead of absorbing them.

Weak Signal Propagation Reporter (WSPR) is an HF beacon system, where stations transmit a digital signal containing your call sign and Maidenhead Gird locator for several seconds. The challenge is to have an efficient antenna and use as little power as possible. In this case about 200 mW (0.2 watts) or 23 dBm. The modulation type is MFSK and the bandwidth is 6 Hz. According to Wikipedia, which is mostly accurate about things like this; WSPR uses a transmission protocol called MEPT_JT. That sends messages composed of:

28 bits for callsign, 15 bits for locator, 7 bits for power level, total: 50 bits.
Forward error correction (FEC): non-recursive convolutional code with constraint length K = 32, rate r = 1⁄2.
Number of binary channel symbols: nsym = (50 + K − 1) × 2 = 162.
Keying Rate is 12000 ⁄ 8192 = 1.4648 baud.
Modulation is continuous phase 4 FSK, with 1.4648 Hz tone separation.
Occupied bandwidth is about 6 Hz.
Synchronization is via a 162-bit pseudo-random sync vector.
Each channel symbol conveys one sync bit (LSB) and one data bit (MSB).
Duration of transmission is 162 × 8192 ⁄ 12000 = 110.6 s.
Transmissions nominally start one second into an even UTC minute: e.g., at hh:00:01, hh:02:01, etc.
Minimum S/N for reception is around –34 dB on the WSJT scale (2500 Hz reference bandwidth).

Distant stations report reception to a database. Several good websites display reception in a map or table format.

This map shows a good path to coastal Maine on 40 meters. The received signal-to-noise ratio is -2 dB at a distance of 423 KM.

80 Meter End Fed Half Wave antenna supported by trees

My antenna is an End Fed Half Wave (EFHW) cut to 3.568 MHz which can be used on any harmonically related frequency (7, 10, 14, 18, 21, 24, and 28 MHz). To accomplish this, a 49:1 Unun (Unbalanced feed to unbalanced feed) transformer is used to transform the 2,400-ohm impedance of the wire to the 50-ohm impedance required by the transmitter. The antenna works best against a ground system that is not less than 0.05 wavelength or 18 electrical degrees on its lowest frequency. That works out to about 4.2 meters (14 feet). A little bit longer is a little bit better. Six 20-foot long 14 gauge bare copper ground radials are attached to an 8-foot ground rod.

Diecast aluminum box containing 49:1 Unun

The Unun is two FT240-52 (not an affiliate link) cores with 14 gauge enamel wire consisting of 2 turns on the primary and 14 turns on the secondary. The antenna is 40 meters (132 feet) of 10 gauge hard-drawn stranded copper wire. This should be good for about 800 watts CW/SSB on HF if I want to use it in that capacity.

Unun wire tied to a DIN rail with 100 pF 5 KV capacitor

There are several guides on how to make the unun available via Google search. There is some debate on whether a 64:1 transformer should be used. Most indicate a 49:1 is the best match. The diecast aluminum (not an affiliate link) enclosure is a nice feature. It cost $33.00 on Amazon.

I used the network analyzer to trim up the antenna a bit. I made a few measurements, the first was just the wire with no ground connected. The next was the wire and ground system after trimming the length for resonance on 3.5 MHz.

The transmission line is LMR-400 with N connectors. I loath PL-259s and use N connectors whenever possible.

I did a series of broadband SWR sweeps. The first was just the wire prior to trimming.

First sweep, frequencies are a little low, SWR is a little high

The next was with a ground rod and six ground radials, #14 bare copper wire twenty feet long.

EFHW trimmed up and looks good on everything except 60 Meters (10 MHz)

This demonstrates the effect of a good ground system. It is worth the effort (and it is an effort) to put in some buried ground radials with this type of antenna. I think above-ground radials would work too.

Here is a screenshot of the little Zachtek desktop WSPR beacon transmitter I bought. This is a great addition to the toolbox and works well for testing the radiation efficiency of an HF antenna. It has a GPS antenna input for timing and location reference. The frequency bands are selectable if you are testing a mono-band antenna. It will work into a fairly poor load, so I suggest sweeping the antenna first with an analyzer.

This shows that my signal is getting out. So far, the furthest distance is 17,030 km with an SNR of -10 (Australia, VK5ARG). That is quite amazing when you think about it. I am letting this run overnight to see how the propagation changes. Overall, this was a good recreational project and now I have a known working HF antenna.

Making a notch filter

One small RF project that I am working on; a 770 KHz notch filter. I always figure if I am having this problem, then others may be having it too. This is a relatively simple idea, a resonant LC circuit (AKA a tank circuit) tuned to the carrier frequency. It should have a bandwidth of +/- 15 KHz of the design frequency. Another requirement; use the parts I have available. Finally, the environment in which this is to be used is a high-noise room; with lots of computers, LED lights, etc therefore it needs to have excellent RF shielding.

Something like this would work well for anyone that lives around an AM transmitter site and is having problems with receiver sensitivity or transmitter intermodulation.

The basic design looks like this:

Time for a trip to the local storage facility known as “The Barn.” In my backyard, there is a small agricultural structure that is used for storage of just about everything. In The Barn, I found several parts salvaged from an old Energy Onyx Pulsar AM transmitter. As such, they are more than capable of receiver operation and could likely handle a fair amount of RF power in the transmit mode.

CDM F2B 0.01 uF capacitor with back of N connector inputs

Finding a type F2B 0.01 uF capacitor, rated at 2000 volts and 11 amps, the value of the inductor was calculated. For the inductor, a 20 uH coil with taps will work great. For receive-only applications, much smaller-sized components can be chosen. Also, there are many bandstop filters with multiple poles. Those are great, but I like the simplicity of the parallel resonant LC circuit.

20 uH inductor salvaged from Energy Onyx transmitter

The N connectors were salvaged from I don’t know where and the enclosure used to house a power supply for a Radio Systems console.

For shielding, I sanded the paint off of the enclosure where the lid is attached and tacked some brass screen down with gorilla glue. This will make a good RF contact surface. The outer of the N connectors are bonded to a piece of copper ground strap which also has a grounding lug on it.

Enclosure lid with brass screen to make contact

I used the Libra VNA to tune it up:

The scan shows it is -31 dB on the carrier frequency. It is -17 dB on 760 KHz and -20 dB on 780 KHz. This is good, because I may still want to listen to the station on the remote receiver. According to the smith chart, it is actually resonant on 771.5 KHz, but that is close enough for this application. I think the resonance went up slightly when I put the cover on after the tune-up.

There are several tank circuit calculators online. It is best to have more capacitance and less inductance to keep the Q of the circuit low and suppress the sidebands as well as the carrier.