VK2XSO posted a sweep of “Return Loss (SWR) (the lower plot) from 500 to 2500MHz of a 50Ω load through ~5m of RG59” apparently to demonstrate his knowledge of transmission line basics. As he says “here are also many other things we can deduce from looking at these two lines.”
For students of transmission lines, some deductions… Continue reading Exploring VK2XSO’s transmission line example
Many years ago I bought a Tigertronics Signalink USB, principally for measurements using FSM and NFM.
A nuisance with the device is the lack of a simple means of disabling PTT. This is particularly important if the system is being used for noise figure measurement with an expensive calibrated noise source which will not withstand transmitter output.
This note describes the fitting of a front panel PTT disable switch. Continue reading Signalink USB PTT disable
Many years ago I bought a Tigertronics Signalink USB, principally for measurements using FSM and NFM.
The Signalink USB turned out to be a disappointment for several reasons but the main one was that the noise floor was almost 20dB higher than an ordinary $10 Creative desktop sound card. Continue reading Signalink USB noise floor improvement
Hams often speak of inline RF wattmeters as being ‘averaging’, but are they? Continue reading Are inline RF wattmeters really averaging
A ham posted online:
I spent several happy hours this weekend building the DE of the 6M Quad described in the June 2014 QST, p 30. When I got it completed, I put the antenna analyzer on it, expecting to find a nice resonance in the 50-51Mhz region and an impedance of 120 ohms or thereabouts. To my surprise, the radiation resistance in the couple of dozen ohm range, and resonance, if that is what I can call it, depends on how am I holding the loop.
After a bit of QST bashing in the thread, he later reveals:
The trial with the analyzer was about 2′ of RG-8X with PL-259s on each end, to BNC jacks on both antenna and analyzer with adaptors.
Much as the chap expressed his lack of confidence in modelling tools, NEC reveals what is happening. Continue reading Analysers – help or hindrance
NFM has been updated to v1.18.0.
It includes for user convenience, a noise measurement uncertainty calculator based on the discussion of uncertainty of the noise sampling process at (Duffy 2007b) and the calculator at (Duffy 2007c)
Much is written about the virtues of some types of coax connectors over others.
Continue reading Coax connectors and accurate / repeatable measurements
A common method of making Noise Figure measurements of a receiver is to use a noise generator of known noise power. The output power of the DUT is measured with the generator off (NoiseLo) and on (NoiseHi), a Y factor calculated, and from that Noise Figure is calculated.
Continue reading Using an attenuator for NoiseLo/NoiseHi in NFM
(Allison et al 2011) detail the method used by the ARRL in their test reports on equipment.
Effectively they calculate NF=-174+27-MDS where MDS is measured in the CW mode using the 500 Hz, or closest available IF filter (or audio filters where IF filters are not available).
!–more–>
One flaw in this method is that the factor 27dB in the NF formula implies that the Equivalent Rectangular Bandwidth
(ERB) of the receiver when measuring MDS is exactly 500Hz. More correctly, the formula should be NF=-174+10*log(ERB)-MDS. The error could be significant, especially with the closest available…
rovision in the test requirement.
For example, above is the measured IF response of a TS-2000 set to 500Hz bandwidth. As filter responses go, it is very good, having quite a flat region, better than most crystal filters used for CW, but the ERB is actually 430Hz, that is 0.66dB less noise than a 500Hz ERB filter and any NF based on 500Hz is low by 0.66dB.
The red plot is that of an idealised filter of the same ERB.
The plot above is for an R-5000 with 500Hz crystal filter. ERB is 446Hz, 0.5dB lower than the nominal 500Hz bandwidth which would lead to an error of 0.50dB using the ARRL’s method for estimating NF.
A more fundamental flaw is uncertainty in measured MDS leads to a relatively large uncertainty in NF when NF is small. The test reports do not specify the uncertainty of MDS, it is unlikely (Duffy 2007), (Duffy 2007b) that with receiver bandwidth of 500Hz (as specified for the MDS test) and the HP339A instrument used, that uncertainty to 95% confidence level is as low as ±0.5dB probably closer to ±1dB. The method used is just not suitable to low noise receivers.
Giving NF rounded to 1dB is not very informative for receivers with NF below 5dB, and fairly useless at 2dB as in (Wilson 2012).
I described a method for designing antenna systems to avoid excessive voltages in baluns and ATUs at (Duffy 2011) .
This article reports post implementation measurements of an antenna system designed using that method and using a G5RV Inverted V with tuned feeder and ATR-30 ATU with integral 1:1 current balun. The tuned feeder is a home-made line section of 2mm diameter copper conductors spaced 50mm, and 9m in length. An additional 0.5m of 135Ω line connects from the antenna entrance panel to the ATU.
