Choosing a feed line for a 70 cm installation
You are installing a 70 cm station. The run from the shack to the antenna is 25 m. You have two cables available: a thin foam-dielectric cable with a published loss of 0.35 dB/m at 432 MHz, and a thick air-spaced cable with 0.09 dB/m at the same frequency. The antenna has 12 dBi gain and the transmitter delivers 25 W. You also need to consider whether the cable can be routed around a sharp bend in a conduit and whether the connector loss matters. The receiver's noise figure is 3 dB and you want to work weak-signal SSB.
1. Calculate the power at the antenna for both cables and explain the practical consequence.
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Thin cable: 0.35 dB/m x 25 m = 8.75 dB total loss. Power factor = 10^(-8.75/10) = 10^-0.875 = 0.133. So 25 W x 0.133 = 3.3 W at the antenna.
Thick cable: 0.09 dB/m x 25 m = 2.25 dB. Factor = 10^(-0.225) = 0.596. So 25 W x 0.596 = 14.9 W at the antenna.
The difference is about 6.5 dB, which is more than a fourfold power difference - roughly the same as going from 25 W to well over 100 W. In ERP terms, with 12 dBi antenna gain the thin cable gives about 3.3 W x 15.8 = 52 W ERP, while the thick cable gives about 235 W ERP. For weak-signal SSB the thick cable is clearly the right choice if it can be installed.
Note that the loss also applies on receive: the same 8.75 dB of loss ahead of the receiver raises the effective system noise figure by that amount, which is far worse than the receiver's own 3 dB. This is often the decisive argument on 70 cm, where cable loss is high and the antenna is the only gain available.
2. The thick cable has a minimum bend radius of 120 mm and the conduit has a 90-degree bend of 60 mm radius. What are your options?
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You cannot force the thick cable around a bend tighter than its minimum radius: the dielectric will deform, the impedance will change locally and the cable may fail or become lossy. Options are:
- Re-route the conduit, or use a larger-radius bend or an inspection elbow, so the bend radius is respected. This is the best solution if the building permits it.
- Use a short jumper of the thin, flexible cable at the bend and join it to the thick cable with low-loss connectors. The extra loss is limited to the jumper length, but each connector pair adds a small loss and a potential fault point.
- Use a flexible cable of intermediate size for the whole run, accepting more loss.
- Move the antenna or the shack so the run is shorter and straighter.
If you use jumpers, keep them short, use quality N or 7/16 connectors, weatherproof the joints and account for the extra loss in your link budget. Do not kink the thick cable to make it fit; a kinked coax is a permanent fault that may only show up as intermittent SWR changes.
3. How would you verify the installation is sound before putting it on air?
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Measure the SWR or return loss at the transmitter end with the antenna connected, and again with a known good dummy load to separate cable and antenna effects. A frequency sweep across 430-440 MHz shows whether the antenna resonates where expected and whether there are narrow spikes indicating a connector fault.
Use a time-domain reflectometer or, more simply, measure the cable loss with a dummy load at the far end and compare against the published figure; an unexpectedly high loss points to a damaged or water-filled cable. Check DC continuity and insulation resistance of the coax, especially if it has been outdoors.
Inspect every connector for correct pin depth, tightness and weather sealing. Then make a real contact with a distant station and compare signal reports with the calculated link budget. If the received signal is weaker than predicted by several dB, suspect a connector or a bend problem before blaming the antenna. Record all measurements so future faults can be diagnosed by comparison.