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:
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.
I bought an inexpensive GPS antenna on eBay, the requirement was for one that operated from 3-5V to suit both of my GPSDOs. The antenna is mounted in an electrical junction box on conduit above the roof for reliable coverage.
The question was whether the active antenna with 5m of RG174 coax and SMA male connector at A$6 posted was any good.
To map its behaviour, it was attached to a Trimble Thunderbolt GPSDO and Lady Heather used to plot rx signal level over about 30h.
Above is the plot. The pattern is not quite symmetric as there are obstructions in play, in fact some of the dips in performance are explained by specific trees and the roof profile. There is a gap to the south at low elevations, GPS satellites don’t fly there (MEO inclined 55° from the equator) .
Overall, it reaches similar strength at the zenith as other antennas tried.
Overall evaluation, it seems to work ok though the coax is a bit rough.
Both cables are of similar size, ~10mm overall, stranded centre conductor and foil+braid outer conductor. The shield stranding is different and the foil is copper in the UF10, aluminium in the LMR400UF.
Let’s take the loss factors calculated for TLLC and de-construct the conductor and dielectric loss for each line type.
A recent long running thread on QRZ entitled “True balanced auto-tuner” was sure to tease out some pretty woolly thinking… the word “true” was enough to signal the outcome.
There are only three words in the title, we can dismiss “true” as a harbinger of woolly thinking, and though people will argue the toss on the appropriateness of the term “auto-tuner’, most people share an understanding of the meaning. “Balanced” is another problem altogether.
After thirty odd posts, there has been no definition or discussion of the term balanced, or its advantages or disadvantages.
One of the recommendations by several posters is the old is new again solution, the once popular link coupled tuner and the work of W5ZQ featured in one of those recommendations.
W5ZQ and WW8J
W5ZQ describes a tuner inspired by WW8J. W5ZQ extended the design and provides a writeup on optimising balance.
Above is W5ZQ’s partial circuit. In the article he describes and shows:
adjustment of the grounding point of the output tank; and
current meters which presumably attach to J2 and J3.
Key to analysis of the topology is that the centre of the output inductor is grounded. This results in the circuit tending towards equal but opposite phase voltages on the output terminals. Continue reading True balanced tuner
Given the lack of activity from credible stations on 40m, it seemed worth a checkout on 30m befor committing to the trial run, large download and data analysis.
So, I ran WSPR for a half hour just before 0000Z and observed the activity on WSPRnet map. I should note that my tx power was 0.1W and rx performance was impaired as there was a 20dB attenuator in line to achieve the tx power.
Above, the map after of the half hour of activity.
An experiment was conducted on 40m using WSPR to compare my own station transmit performance with others relatively close by.
The experiment was conducted around sunset on 01/08/2017, data was collected for the period 0600Z to 0900Z, sunset was at 07:17Z.
The experiment was unannounced as previous experience has been that if the WSPR community becomes aware of activity that does not accord with individual’s opinion of acceptable, the activity can be disrupted.
Data for analysis was fetched by downloading the archive which contained nearly 1,000,000 records for the day, and about 340,000 of those were 40m spots.
Factors shaping experiment design
The following is a discussion of various factors that weighed into experiment design.
Transmitters tend to cluster around the centre of the 200Hz WSPR band.
Above is a frequency distribution of tx frequency, and it is evident that the risk of interference is reduced by choosing a frequency near the upper or lower limit of the band segment. There was some activity just outside the designated band segment which might indicate care and competence of operators.