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
- the balun works well on a nearly symmetric load.
Continue reading G3LNP balun with symmetric ‘matched’ load
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
This article shows just how easy it is to make an inexpensive low VSWR load for antenna analyser validation / measurements.
Above is an AA-600 sweep of the prototype from 10kHz to 100MHz. VSWR reads 1.02 in ‘All’ mode at 100MHz… better than the inherent accuracy of the instrument.
It is made from two 100Ω 1% 1206 SM resistors purchased on eBay for about $2/100, so about $0.04 for the resistors, and 40mm of bare copper wire (0.5mm phone / data wire in this case).
In use, it is held in contact with the coax socket (in this case an N type) with a pair of disposable plastic first aid tweezers (yep, you can buy them on ebay for about $0.20/pair).
While you are at it, make a good short circuit termination by scrunching up a bit of (clean) kitchen aluminium foil and press that against the coax socket conductors.
Try both of these on your antenna analyser and see how it stacks up.
DL4YHF published a frequency counter design based on the PIC16F628.
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.
Above the built kit with the 7550 voltage regulator replaced with a 78L05. Continue reading DL4YHF 50MHz counter on a Chinese TB-244746 PCB
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 (0.005,13).
- source has a Thevenin equivalent source impedance of 50+j0Ω;
- feed line is lossless.
The results are sensitive to the model assumptions.
We will calculate the ratio of radiated power to the power delivered by the transmitter to a matched load, let us call it TransmitEfficiency for the purposes of this article. Continue reading “No short antenna is more efficient than a resonant-length antenna”
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).
The test was carried in a few spots, at 50-100 m from the main station dipole. Continue reading Small untuned loop with MMIC LNA on 80m
A ham recently posted a graph on QRZ to educate his fellow hams on the behaviour of transmission lines under mismatch.
Above is one of his graphs (the red arrow is my annotation). It plots Impedance variation along a mismatched 75Ω transmission line. The curves look graceful, but are they science or just pretty artwork? Continue reading Failure to treat impedance as a complex quantity leads to…
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:
Key metrics are:
- ReactanceDial offset=200
- Feed line is 4m of Belden 8262 (RG58C/U).
Continue reading Demonstration of the GR1606B for antenna Z measurement and calcs
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.
Continue reading BG7TBL noise source
An online expert recently reported:
I tried to make an antenna loop for longwave with cat 5 and after it did no good I realized the twisted wires canceled each other out.
Or did they really cancel?
I constructed a loop of one Cat 5 pair and measured its inductance when both wires are bonded at the ends.
The conductors are 0.5mm diameter and spaced 0.9mm. To estimate the inductance we use the geometric mean radius (GMR) as the equivalent radius of the pair. GMR=(0.5*0.9)^0.5=0.67, diameter=1.34mm. So let’s calculate the inductance of a single turn circular loop of 0.8m perimeter and round conductor of 1.34mm diameter.
The estimate above is 850nH.
Above is the measurement, the screen is not readable, but it is 852nH, very close to the estimated 850nH. Continue reading Inductance of a loop of CAT5 pair