A correspondent recommended a simple configuration of a base loaded shortened Marconi for 137kHz, referencing an online posting by another ham.
I was assured that this configuration is simple, very effective and very popular. It has been used for a very long time, so it must be good.
Well, let’s do an analysis.
The recommended antenna
The online poster’s equivalent circuit of his 137kHz base loaded vertical. The resonant frequency of this circuit is actually 136.979kHz, let’s assume the inductance is correct and that C is C=99.969pf and the circuit is resonant at exactly 137kHz.
NEC model
Since it is base loaded, we can build an NEC-4.2 model of a base loaded vertical where the loading inductor for resonance is 13.5mH as per the circuit above.
For this model, let’s reuse an existing model that has 8 x 40m long elevated radials (100mm height) over average ground (σ=0.005, εr=13), and adjust the vertical radiator length until it resonates with the specified loading inductor. Then Q of the loading inductor is adjusted until the combination is matched (as shown in the schematic above). This technique is trading (sacrificing) radiation efficiency for impedance match .
Above is the prototype antenna geometry, and current magnitude is plotted in green.
Feed point impedance of the unloaded antenna is 4.8-j11621Ω.
When matched the generator sees an impedance of 50+j0Ω, comprising:
- Radiation Resistance 0.008Ω;
- structure loss resistance 0.032Ω;
- ground resistance 4.76Ω; and
- coil loss resistance 45.2Ω.
One could create a pie chart of the resistances, but the presentation will not be very informative as the last two will dominate, the first two will be invisible to the naked eye.
Q of the inductor can be calculated as 11621/45.2=257… not very good.
System radiation efficiency is 0.0165% or -37.8dB.
Maximum gain is -33.3dBi. To achieve the licence limit of 1W EIRP, approximately 2100W will need to be delivered from the source.
Does it live up to the claims?
The claims were:
- simple;
- very effective; and
- very popular.
The standout failure here is “very effective”, it has appalling radiation efficiency, about 16dB worse than A desk study of a matching scheme for a cap hat loaded Marconi on 137kHz and there are several reasons for that, but 16dB performance drop is a big offering in the interest of simplicity and popularity.
This example shows that good VSWR does not imply good radiation efficiency.
One wonders whether proponents of this type of antenna have measured far field strength and evaluated system radiation efficiency. I would of course defer to credible measurement.
That said, inefficiency in transmit mode can be compensated by running higher transmit power, and this might require 1kW or more to achieve the VK power limit of 1WPx EIRP.
Now for receiving where external noise level is very high, one does not need an efficient receive antenna for low Signal to Noise Degradation, so this might be quite effective for receiving. Adequate receiver performance might be taken incorrectly to imply adequate transmit performance.
Conclusions
It is possible to resonate base loaded shortened vertical with a series inductor of reactance such that it cancels the -ve reactance of the radiator.
Focusing on VSWR as the only indicator of performance is naive.
Loss in matching components degrades radiation efficiency, and the degradation can be large.
The loading coil brings its own loss resistance which is added to the base resistance of the radiator (itself the sum of radiation resistance, structure loss resistance and equivalent ground resistance). If the sum of these is very close to 50Ω, you have an apparent ‘perfect match’. ‘Building out’ with loss is exploited in some antenna system designs.
Receive performance is evaluated differently to transmit performance, and in the presence of very high external noise, a low gain (inefficient) antenna might still achieve low Signal to Noise Degradation.
Traditional wisdom bears another look, question everything.