Hams often would like to know the impedance of an antenna at its feed point, sometimes for very sound reasons, and very often in pursuit of a specious goal.
One of the oft given suggestions is that it is convenient to measure through an integral number of electrical halfwaves of transmission line, since as everyone knows, impedance at the end of the line is repeated exactly every half wave towards the source.
Some even tell us that they cut their feed line lengths to exactly nλ/2 to facilitate this at implementation an into the future. So, lets take that idea and cut the feedline to the shortest nλ/2 that will reach the feed point 100m distant. The electrical length of a VF=0.83 feedline will need to be nλ/2 or 1080° at our nominal frequency of interest, 7.2MHz.
To explore the method, let’s use the modelled feed point impedance of a 40m Inverted V Dipole used in some recent articles.
The real feed point
Above is a Simsmith model of the feed point impedance, The blue line overlays the magenta line which is the locus of s11 from the NEC model. Continue reading On measuring antennas through integral halfwaves of transmission line
I have seen several online posts of hams citing measurement of phase of s11 as a figure of merit of a matched antenna system, indeed evidence of the resonance nirvana.
Let’s review the meaning of s11.
s11 is the complex reflection coefficient at the reference plane, usually wrt Zo=50+j0Ω.
If you were to measure the s11 looking into an ATU, you might adjust the ATU to minimise the magnitude of s11 (|s11|) which is also minimises VSWR. If you do a really really good job of adjustment, you might achieve around the noise floor of the instrument.
You can simulate this near perfect match by simply sweeping your 50Ω calibration load. Let’s do that and look at some relevant views.
Above, |s11| expressed in dB is very low, it is at the noise floor of the calibrated instrument, and it is very jittery… due to the relatively high contribution of noise. Continue reading On the measured phase of s11 in a matched system
This article describes an antenna system for 40m based on:
- an inverted V dipole;
- Guanella 1:1 balun; and
- a ‘tuned’ length of RG11A/U CCS coax.
The antenna system will be centred on 7.080MHz to suit my own operating preferences.
The coax is that featured at Checkout of a roll of Commscope 4510404 CCS RG11A/U – Zoc, Zsc based MLL calculation and the matched line loss is taken from measurement as 1.2dB/100m @ 7MHz (all conductor loss). The feed line cost $99 for 305m incl delivery, so this project uses $6.50 worth of cable. The feed line is not good because it is cheap, it is good because it suits the application very well, and as a bonus, it is inexpensive.
The broad concept is that the dipole is tuned a little shorter than a half wavelength to excite a standing wave on the coax. The VSWR desired is a little over 1.5, and the length of the coax is tuned so that the impedance looking into the coax is close to 50+j0Ω. “A little over” is so that the VSWR at the source end is very close to 1.5.
Above, the topology of the Inverted V Dipole with modelled current distribution in green. The apex of the dipole is at 11m and it is over ‘average ground’ (σ=0.005 εr=13). Continue reading Applying the RG11A/U to a 40m Inverted V Dipole antenna
VK2TP at Wellington has been making observations of aircraft enhancement of the path between my 144MHz beacon and Wellington, an obstructed direct path of 259km. The beacon modulation pattern includes a long steady carrier which permits better observation of the nature of these aircraft enhanced paths. Continue reading 144MHz beacon observation from Wellington NSW – 3 aircraft reflections
The article Checkout of a roll of Commscope 4510404 CCS RG11A/U – Zoc, Zsc based MLL calculation ended with a comment on making the measurements with a nanoVNA.
This article reports measurements with a nanoVNA-H v3.3 (modified) calibrated and swept from 1-31MHz using nanovna_mod. Continue reading Checkout of a roll of Commscope 4510404 CCS RG11A/U – Zoc, Zsc based MLL calculation – nanoVNA
One sees discussions online regarding the suitability of various conductor materials for antennas and feedlines.
A recent thread asked about
steel as an antenna radiator and one response tried to answer the question using NEC models of a half wave 40m dipole.
Above is the graphic offered to support the analysis that steel is only 0.5dB behind copper, the poster stating: Continue reading Steel as an antenna radiator
Checkout of a roll of Commscope 4510404 CCS RG11A/U documented a simple and quick test of the matched line loss (MLL) of a roll of new RG11A/U just delivered. The test used is simple, and quite suited to the case where the far end of the cable is not accessible, eg buried inside the drum.
In this case, the far end of the cable was accessible and a series of measurements of impedance with a short and open termination could be made. This is a more comprehensive test, but involves more complicated calculations. Continue reading Checkout of a roll of Commscope 4510404 CCS RG11A/U – Zoc, Zsc based MLL calculation
I saw an eBay listing for 305m (1000′) rolls of Commscope 4510404 CCS RG11A/U which looked interesting, time for some due diligence.
Being a brand name product, specifications were probably available and indeed were quickly found.
The cable is a CCS RG11A/U (Zo=75Ω), and from the centre conductor DC resistance value (12.5Ω/305m) and 1.63mm (#14) size, it appeared likely that it was 21% IACS Copperweld, and that the cladding thickness should be 49µm. At that copper thickness, near copper like performance was likely down to perhaps 7MHz (where it is 2 skin depths thick).
A roll was ordered as it was likely to be quite usable cable and at $99 including shipping (~25kg) it should be good value. Continue reading Checkout of a roll of Commscope 4510404 CCS RG11A/U
The technique is to make a measurement near to the frequency of interest of Rin at resonance of the length of line with open circuit at the far end, and to calculate the matched line loss (MLL) using Calculate transmission line Matched Line Loss from Rin of o/c or s/c resonant section.
Let’s demonstrate the measurement of Rin of an o/c resonant section around the 160m band, which we will then use to calculate MLL.
Above, the AA-600 connected to the cable using a F(F)-N(M) adapter, the cable is 305m in length and the far end is open circuit. Continue reading A walkthrough of using a Rigexpert AA-600 to make a quick measurement of loss of a new roll of CCS RG11
A model of current distribution in copper clad steel conductors at RF described a simple analytical model. This article compares the results of that model with graphs published in (Severns 2000) and is of particular interest since Severns used an FEA based model.
Above is Fig 3A from Severns and is the current distribution at 1MHz for 20.3µm (0.0008″) cladding of a 0.405mm (#26) CCS (Copperweld) conductor. Continue reading A model of current distribution in copper clad steel conductors at RF – comparison with Severns 2000