The similitudes and
differences between the
Miniwhip active E-field antennas.
The Hi-Z input circuit of the impedance matcher electronics in a Miniwhip antenna, detects the voltage differences between the antenna surface and its mass-plane.
These RF voltages will become available at the 50 Ohms coaxial output.
Why a coax feeder can act as a parasitic antenna.
Between receiver and antenna, signal and noise sources will induce RF currents on the outer skin layer of the coax screening ("its Common Mode Circuit").
These "Common mode noise currents" will travel to the antenna.
They there can enter the internal coax circuit (its differential mode circuit).
These noise signals then run through the inside of the coax backwards to the receiver.
And will there be heard.
These common mode noise currents will deteriorate the "Signal To Noise Ratio" of the antenna system .
Less common mode currents will result in improved Signal To Noise Ratio.
Measures to improve a Miniwhip S/N.
- The bottom of the Miniwhip mast MUST be DIRECTLY, VERY SHORT and NOISE FREE grounded into the soil.
- The Miniwhip PCB ground surface should be directly connected to the top of that grounded mast.
- The shielding of the coax feeder should be connected to that grounding point at the bottom of the mast.
This will bleed common mode noise currents [Icm1] toward ground, before they can reach the antenna
PCB. This measure is effective for a wide band width.
- Insert every 3m distance a good common mode choke over the coax.
They will prevent common mode resonaces.
But will be less or not effective at VLF/LW/MW bands.
On VLF/LW/MW and low SW bands, the top of the grounded mast is still low-Z, as the
length of the mast is short compared to the wavelengths.
But on higher SW frequencies, for instance 29MHz and a (1/4 wave) 2.6 meter long mast, the mast resonates at 29 MHz. The mast top is then highZ.
Coax common mode currents running through the mast will then cause RF noise voltages at the top of the mast, possibly worsening the Signal To Noise Ratio on higher frequencies.
The weaknesses of every active E-field antenna :
- Noise free grounding of the antenna and feeder is absolutely necessary.
But not always possible, if no free piece of ground (garden) is available. No ground => poor performance !
- Screening of the antenna is impossible.
- Balancing of the antenna is impossible.
- They are VLF/LW/MW and SW DX antennas.
- But they produce WEAK NVIS signals (sources from distances less than 400 km between 3 MHz and 8 MHz will be 20dB weaker).
My Miniwhip antenna PCB :
Contains a Hi-Z resistive Common Mode Choke (CMC), inserted between the electronics output, and the antenna output BNC bus. The CMC impedance is abt, 5 kOhms resistive above 2 MHz.
It will over a wide bandwidth vastly reduce common mode currents running to the mast, or jumping into the inside of the coax.
: To prevent a ground loop, the outside of the BNC output bus may NOT be
It MUST be left "floating".
Only the one PCB grounding point must be connected to the top of the mast.
My version of the LZ1AQ active wideband receiving loop antenna has better properties than active wideband E-field antennas.
- Grounding of
my version RXloop antenna unit is NOT needed, nor advised. =>
Simple antenna setup.
- Loop antenna, loop amplifier and splitter are all fully shielded. Common mode noises cannot intrude electronics.
- Loop antenna, loop amplifier and coax feeder are fully balanced. Common mode noises will be suppressed.
- The balanced shielded loop caries at both terminals equaly strong and in phase common mode noise currents.
- The balanced input of the loop amplifier will suppress these common mode noises by at least 30 dB.
- The output of the antenna amplifier is also balanced, transformer coupled, including a CMC in its output.
- Due to the fully balanced and fully screened construction, my loop antenna common mode suppression remains very good down to 15 kHz VLF.
- This is a vertically standing loop, beside low incident (DX) signals, it also
- By turning the loop around its vertical axis, it can suppress low incident signals by more than 30 dB (! very sharp) .
- CMCs every 3m over the coax prevent 1/2 lambda resonance and amplified noise regions.
- Common mode noises are vastly suppressed.
- Compared with some active loop antenna webSDR stations, resulting also in far weaker thunder storm lightning noises.
|The balancing and screening measures proved to be the most effective for S/N improvement.|
The common mode suppression in my RXloop is so high, that it can only be concluded observing very
strong, over -50 dBm, signals or noises.
When the power to the antenna is switched off, less strong signals will vanish below the receiver noise floor (equivalent to -120 dBm).
Why so much CMCs over the feeder ? Overkill ?
articles K9YC suggests :
- Asymmetrical antenna systems and 1/2 wavelength long feeders cause high voltages on the antenna and transmitter.
- Use a symmetrical antenna, and install a good CMC at its feed point.
- Install a CMC at the transmitter (or matcher).
- Install a CMC halfway the feeder.
concluded : this is also valid while receiving.
I discovered that
feeder 1/2 lambda
common mode resonance's can cause amplified noise levels, around
frequencies where the feeder length is a 1/2 lambda, or a multiple of a
How to diagnose common mode resonance effects :
Do you suspect this phenomena, than you can test it by temporarily inserting an extra piece of feeder coax.
If the suspected noise "hills" shift down in frequency, they are caused by a Common Mode 1/2 lambda resonance.
Valid for all active wideband receiving antennas :
Install CMCs over the feeder coax at distances of ~1/4 lambda for the highest frequency used.
For instance :
- Highest frequency : 30 MHz,
- Wavelength : 10 meters.
- Multiple CMCs inserted on the feeder at distances of about 3m .
REM : Start with CMCs at the receiver, and the active antenna unit.
Fill the feeder with CMCs in-between.
result will be, that no common mode resonance's, nor amplified noise hills in
the waterfall screen can occur
below 50 MHz.