This article has been copied by request from my VK1OD.net web site which is no longer online. The article may contain links to articles on that site and which are no longer available.
(Tester 2013) described a coaxial collinear array for VHF/UHF. Tester describes the antenna a collinear is a vertical antenna whose resonant elements are connected along a common line (ie co-linear) so that each element is opposite in phase to its neighbour.
He is a little confused, in fact, the elements are in-phase with each other so that in the horizontal direction, the contribution of the current in each element to the far field is an additive or reinforcing one.
He goes on to say [i]f you are not into antennas, that mouthful is, fortunately, very easy to achieve… but is it?
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.
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.
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.
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.
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.
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