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).
A guanella 1:1 balun is incorporated in the Simsmith model by way of 1m of lossless 110Ω transmission line with VF=0.9. This is to model a balun wound with twisted pair, eg the balun shown above which is wound with XLPE insulated wire which is lower loss and better voltage withstand than PVC.
Let's look at the form of the solution in Simsmith, well it is the actual solution but we will talk just about the form of the solution first.
Above, the specified load is the impedance looking into the dipole centre from the NEC model. T1 is a length of nominally 75Ω transmission with some loss (1.2dB/100m @ 7MHz as per previous measurement), note the spiral due to line loss. T2 represents the balun by way of 1m of 110Ω transmission line with VF=0.9. So this is the form of the solution, a shortened dipole with some significant -ve reactance such as to launch a VSWR(75)≈1.5 standing wave on the 75Ω line, and the line is cut where the impedance looking into the line is 50+j0Ω. From the chart, feed line loss under the specific mismatch conditions is 0.276dB, or about 6%.
Now to the NEC model to design the antenna.
Iteratively it was found that a leg length of 10.03m gave the desired input impedance for the desired input end VSWR on the 75Ω feed line.
Above, a summary of the model.
Above is the gain pattern from the model, but note that there is a small loss in the feed line not included in the model. The pattern is typical of a low Inverted V Dipole, maximum gain at the zenith.
In a world where the only thing of merit is DX, then a flat top dipole has a slightly better gain at low elevation… but if you don't want to hear the DX contesters then the ‘cloud warmer' pattern has relative advantage of ‘local' contacts.
Let's look again at the Simsmith model swept over a frequency range.
The load impedance is exactly that drawn from the NEC model (in fact it is imported from the NEC output file), and the feed line configured to match the Commscope 4510404 CCS RG11A/U. The feed line length was adjusted to minimise VSWR(50) looking into the line.
To find the SYSTEM radiation efficiency, we must multiply the NEC model radiation efficiency (70.04%) by the transmission line efficiency (93.8%) to obtain 65.7%. That is a pretty good system radiation efficiency for an antenna of this type, and there is only 6% of power lost in the 20m of quite low cost feed line.
Above is the VSWR curve from the Simsmith model, VSWR 1.5 bandwidth is over 200kHz, so it is quite a practical matching scheme.
The antenna can be used without an ATU, operationally an advantage and avoiding the need for a costly ATU if high power is involved. Whilst QRP ATUs are less expensive, they also tend to be less efficient… so you achieve QRP^2 when using them.
Note that this is designed as a single band antenna, the matching scheme uses a ‘tuned' length of coax.