This article describes a Guanella 1:1 current balun which has high common mode impedance (Zcm) and low Insertion VSWR. It is for application on antennas that have low VSWR(50) on at least some bands, especially if they would be used without an ATU on some bands.
The purpose of the balun is to minimise common mode feed line current which may contribute to EMC problems when transmitting, and contribute to increased ambient noise when receiving. Reduction of feed line common mode current also helps in achievement of expected load impedance characteristic, radiation pattern and gain. This article gives measured Zcm, but the definitive test of the effectiveness of such a balun is direct measurement of common mode current Icm… and it is so easy.
Example applications are half wave centre fed dipoles, fan dipoles, trapped dipoles, G5RV with hybrid feed, ZS6BKW, trapped verticals, monopoles, ground planes.
To obtain low Insertion VSWR, the choke will be wound with 50Ω coax, to demonstrate the practicality of the design budget (but good quality) regular (ie solid PE dielectric) RG58C/U will be used. Foam dielectric is NOT recommended. Solid PTFE coax could be used, but avoid coax with steel cored inner conductor, it may be lossier than you think at low frequencies with the silver cladding is relatively thin.
The candidate core is the readily available FT240-43 (Fair-rite 2643803802, 5943003801), it is a low cost NiZn ferrite with medium µr, and its µr and loss characteristic contributes to a broad high impedance choke well suited to this application.
My article G3LNP balun explored the operation of the G3LNP 4:1 balun on a 200Ω asymmetric load and found it exhibited extreme Insertion VSWR on what should have been an ideal impedance transformation but for the asymmetric element.
The balun is in fact a Voltage Balun and cannot be expected to work properly on asymmetric loads.
A correspondent proposes that the balun probably works very well on a nearly symmetric load such as a half wave dipole.
There are two aspectes to this proposition:
the assumption that a common half wave dipole implementation is nearly symmetric; and
G3LNP described a 4:1 balun for HF antennas in Radcom Nov 2017.
Above is the schematic supplied by G3LNP. He describes the dashed link at the bottom as optional, but uses it in his prototype so this analysis is with that link installed. The prototype used equal lengths of coax (1m PF100, an RG-6 like coax), and the toroidal choke appears to be 8t on a T130-2 powdered iron core.
Exploration of behaviour of baluns on extreme asymmetric load often reveals whether they work properly for asymmetric loads. Continue reading G3LNP balun
The design has been modified by many, copied by even more, and usually without attribution.
This article documents one of these copies (TB-244756 printed under the chip footprint), a $6 kit off eBay which comes with no documentation, though the screen mask is enough to correctly place components.
It turns out to be DL4YHF’s “DISPLAY_VARIANT 2”, the variation is that it uses a common cathode display.
A recent online posting asserted that an antenna is optimal when itself resonant, and fed with a resonant feed line length so delivering a purely resistive load to a source, and further that implementors needed to be careful that a shorter dipole could be offset to some extent by a longer feed line but it would be inferior because:
no short antenna is more efficient than a resonant-length antenna
… but does that stand scrutiny?
An NEC experiment
Lets walk though an experiment using NEC-4.2 models of a dipole of 2mm copper wire at 10m height at 7.1MHz over average ground (σ=0.005, εr=13).
source has a Thevenin equivalent source impedance of 50+j0Ω;
feed line is lossless.
The results are sensitive to the model assumptions.
This article documents a small experiment with a quite small untuned loop, and LNA and receiver on 80m to assess its ability to copy signals on the band as well as the station transceiver on large antenna.
A significant factor at 80m is that ambient noise is quite high. Let’s consult ITU P.372-12 for guidance.
Curve E is the median city noise, at 3.6MHz Fa is about 62dB. At a more detailed level, P.372-12 gives the median noise figure for Rural precincts at 51.8dB and that figure is more appropriate to the test location (large block rural residential).
This article demonstrates use of a GR1606B RF impedance bridge for measurement of the feed point impedance of a MHz loaded mobile whip. The antenna is roof mounted on a vehicle and measurements are made looking into 4m of RG58C/U, then transformed to feed point impedance using three tools:
This is a review of the BG7TBL noise source available on eBay for about $20 incl post. I have seen this recommended in various online forums and thought it worthy of review.
A quick mention of Excess Noise Ratio (ENR), it is a commonly used measure of the characteristic of noise sources. A noise source for testing low noise RF amplifiers needs to be less than 10dB, 5dB is common; for other receiver testing around 15dB is common, and for massive output for filter alignment etc the noise needs to be well above a spectrum analyser noise floor so an ENR of 50dB might be appropriate (but such high noise output makes it useless for LNA noise figure measurement),
Above is the device. The layout is pretty simple, it is a Zener noise source at the left followed by three MMIC amplifier stages. The circuitry at mid left is a DC-DC converter to supply 25V to the Zener.
There are a host of aspects so far that are concerning:
there is no need to operate the Zener at such high voltage;
lack of regulation of MMIC power supply;
the noise output of the Zener source should be quite high; and
three stages of MMIC will give rise to huge output, notwithstanding the on-board attenuators at Zener output and final MMIC output.