At Yet another dodgy coax collinear I wrote about a design published recently in QST.
More recently, a ‘design’ has emerged in eHam forums that appears to be identical. Continue reading Yet another dodgy coax collinear II
Category: Antennas
An A/B comparison of a low G5RV with a MobileOne M40-1 – statistical analysis
This article reports statistical analysis of the measurements made for An A/B comparison of a low G5RV with a MobileOne M40-1 (read it first).
I left it for readers to visually form a view of the difference between the antennas, and the implications for credibility of folk lore about the two antenna types, this article addresses the quantitative difference between the average S/N ratio of the antennas.
Statistical analysis
Standard statistical techniques can be used to arrive at a difference in the mean S/N of the antennas and to quantify the uncertainty in that statistic. Continue reading An A/B comparison of a low G5RV with a MobileOne M40-1 – statistical analysis
NEC-4 vs NEC-2 on a low small transmitting loop
This article compares a series of models of a small transmitting loop at varying height above real ground using NEC-4 and NEC-2.
The models are of an octagonal loop of thin wire of the same area as a 1m diameter circle over real ground (0.007/17). Height is measured to the centre of the loop, and all impedances are wrt the main loop.
Above is the NEC-2 result.
Continue reading NEC-4 vs NEC-2 on a low small transmitting loop
An A/B comparison of a low G5RV with a MobileOne M40-1
This article reports measurements on 40m of S/N as a receiving station in Melbourne of a 10W transmitter switched between a G5RV and M40-1 over a 1 hour period.
The experiment compares the antenna over a specific, but very relevant ionspheric path so it is more useful than ground level measurements in a car park or playing field.
Above is a screenshot of the beacon signal switched between the two antennas. Continue reading An A/B comparison of a low G5RV with a MobileOne M40-1
Accuracy of estimation of radiation resistance of small transmitting loops
A simple formula exists for calculation of radiation resistance of a small transmitting loop in free space. The derivation is in most good antenna text books.
\(R_r=\frac{\mu_0c_0}{6\pi}A^2(\frac{2 \pi}{\lambda})^4\\\)The expression depends on an assumption that current around the loop is uniform, so the question is what is the acceptable limit for loop size.
NEC might provide some guidance. A series of NEC-4 models of a octagonal loop of thin lossless wire in free space was constructed with varying perimeter. Perimeter shown is that of a circle of the same area.
Above is a comparison of the two methods of estimation of Rr. To the extent that we trust NEC-4, the graph indicates that error in the simple formula grows quickly for loop perimeter greater than 0.1λ. (The results using NEC-2 are visually identical.)
Many authors set the criteria for a small loop to perimeter<0.3λ, but it is clear that current is not sufficiently uniform for perimeter>0.1λ for estimation of Rr as 31149*(A/λ^2)^2 to 0.1pu error or better.
Small transmitting loop – ground loss relationship to radiation resistance
This article documents a series of NEC-2 models at 7.2MHz of a lossless small transmitting loop near ground for the insight that they might provide about underlying loss mechanisms.
Key model details:
- lossless conductor 25mm diameter;
- octagon of sides 403mm, has same enclosed area as a 1m diameter circle;
- three ground types;
- height varies from 1.5-10m to centre of loop.
Impedance elements discussed in this article are referred to the main loop. Continue reading Small transmitting loop – ground loss relationship to radiation resistance
Effect of ground on HF horizontal dipole efficiency – more detail
This article expands on Effect of ground on HF horizontal dipole efficiency with some more model detail for the technically minded. See the original article for model details and discussion.
Above is the efficiency curve expanded to 80m height, about 2λ. The graph assumes no matching loss (mismatch loss, changed line loss). efficiency may be significantly poorer if not matched efficiently. Continue reading Effect of ground on HF horizontal dipole efficiency – more detail
Transmit performance of 2m hand held transceivers – absolute gain estimates
Transmit performance of 2m hand held transceivers reported relative field strength measurements for some transceiver / antenna combinations.
This article documents a more careful measurement of the absolute field strength of one combination, and application of that knowledge to the other results.
Measurements of field strength were done with Lou Destefano’s (VK3AQZ) VK3AQZ RF power meter (RFPM1) and a small loop antenna.
Above, the RFPM1 RF power meter.
Above is the small loop used for field strength measurement. It is 2mm hard drawn round copper wire formed into a circle 185mm in circumference, and a common mode choke is used to connect the loop to the RFPM1 power sensor. The common mode choke is 0.6m of RG58C/U with 0.5m of ferrite sleeves over it and its loss is accounted for in the “Other Loss” item.
Continue reading Transmit performance of 2m hand held transceivers – absolute gain estimates
An interesting case study – in service evaluation of coax loss
A poster sought advice of the forum experts about in service evaluation of the loss of some coax feed lines…
Has anyone tested old coax cable to see if the loss increased over time? I just tested two different coax cables at 146 Mhz with the use of a Bird Model 43 Wattmeter. Power measurements were taken at the input of each cable followed by the output. The load in both cases was a 146 Mhz Ground Plane.
The test results seem to show losses similar to new coax although Berk-Tek foam coax may have had a lower loss when new.
1. Berk-Tek 6211 RG-8X Ultra Flex Foam Coax – 68 feet
Measured 25 watts in and 11.7 watts out which represents a 3.3 db loss. …
Assuming that the stated measured power is in fact the indicated forward power on the Bird 43 directional wattmeter and given that the actual Zo of the line should be very close to the calibration impedance of the Bird (50+j0Ω), then the Matched Line Loss (MLL) is very close to 10*log(PfIn/PfOut)=10*log(25/11.7)=3.3db which is significantly above the expected 2.6dB for ‘ordinary’ RG-8/X and warrants re-measurement as it suggests that the cable might have degraded a little. In fact, the OP later reports 10.7W out for 25W in which is MLL of 3.7dB against spec of 2.6dB… a more convincing case for replacement! Continue reading An interesting case study – in service evaluation of coax loss
Matching a quarter wave monopole with two variable caps
Two recent correspondents have discussed matching a quarter wave monopole with two variable caps.
Two capacitor shunt/series match
The matching scheme involves a shunt variable cap at the end of the coax feed line, and a series variable cap to the monopole base. The radials are of course connected to the feed line shield.
This type of matching scheme requires that the monopole feed point has sufficient +ve reactance, ie the monopole is longer than resonant. Lets assume the R component of feed point Z is 35Ω.
This scheme incorporates the simple shunt match, and the value of the shunt capacitor can be found knowing the R value to be matched to 50Ω.
Above is a Smith chart of a model of the match at 14MHz. The monopole has been lengthened to have 100Ω reactance along with 35Ω resistance. In this case a series cap of 148pF and shunt cap of 150pF are required. Continue reading Matching a quarter wave monopole with two variable caps