Testing VHF Coaxial
The loss in RF coaxial cable increases substantially and quickly, when there is water intrusion. Coax that uses foam dielectric, like RG8X and LMR type coax, is particularly prone to this problem because the water can quickly propagate along the foam dielectric used in these type coaxes. We delve into the causes of this degradation in more detail in the main aritcle on this topic: "Special Report: How to Prevent AIS and VHF Antenna Malfunction."
The lightweight versions of LMR coax (LMR-LW) have another problem when wet, which is an aluminum shield which corrodes very quickly. Simply measuring SWR from the base of the mast is not able to detect lossy coax because the increased attenuation due to lossy coax is indistinguishable from a well-matched antenna.
A relatively convenient solution is to disconnect the antenna at the masthead and then measure the “return loss” which is the amount of RF power reflecting back down the coax from the open end. Because the top of the coax is disconnected, there will be 100-percent reflection, so the coax loss is half the measured return loss.
This test requires the use of a SWR or a directional power meter (often combined in one unit). The tests are the same with a directional power meter and with an SWR meter, you just get different numbers which can easily be converted to determine loss. Inexpensive SWR directional power meters like the MFJ-812B ($40) will work so long as they cover the 156-174 MHz band. The Shakespeare ART-3 is a bit overpriced but is widely available at West Marine.
We use the more expensive Bird 43 power meter. To use a directional meter, you’ll need a 25C “slug” dummy load (widely available for about $30). You’ll also need:
• VHF radio, 25 watts.
• Coax connectors and patch cables for interconnecting the radio, the meter, and the coax.
SWR Meter Field Test
This is the most common test because SWR meters are widely available, inexpensive, and easy to use.
1. Disconnect the antenna at the masthead.
2. Connect the VHF radio to the SWR meter, and the other port of the SWR meter to end of the coax that was connected to the radio.
3. Repeat with antenna connected.
If the SWR is >4.0 with the antenna disconnected, then coax loss is acceptable (i.e. less than 2.2 dB loss).
If the SWR is < 1.5 with the antenna re-connected then the antenna and coax are ok.
Directional Meter Field Test
1. Follow steps 1 and 2 above.
3. Measure the forward power (from the radio to the coax).
4. Measure the reflected power (from the coax back to the radio).
5. Compute the coax loss in dB as 5* (log (P(fwd)/P(ref))). The reflected power “P(ref)” should be greater than 36 percent of the forward “P(fwd)” power. For example if the forward power is 25 watts, the reflected power should be greater than 9 watts, which is 0.36*25.
6. Compare the measured coax loss to the cable specifications and to the requirements. The coax should have less than 2.2 dB (i.e. 40 percent) power loss.
7. Finally, to test the antenna, reconnect the antenna at the mast head. Again measure the reflected power at the boat end of the coax. Compute the SWR from the nomographs that come with the directional power meter. The SWR should be 1.5 or lower. A higher SWR indicates that the antenna or coax is open or shorted.
To test shorter lengths of coax on a bench use the following approach which requires a 50 ohm, 25 w (or more) RF dummy load, readily available from radio stores for around $40.
1. Connect the VHF radio to the directional power meter, and the other end of the directional power meter to the coax.
2.Connect the dummy load to the far end of the coax.
4. Measure the forward power from the VHF radio to the coax.
a. Note the reflected power using he directional power meter. If there is any measurable reflected power it indicates a bad connection or flaw in the coax
5. Move the directional power meter to the far end of the coax, between the coax and the dummy load.
6. Measure the forward power from the coax to the dummy load.
7. Compute the coax loss in dB as 10* (log (P(radio)/P(load))). The math-averse can use Stan Honey's handy online calculator.
8. Comparison between the measured loss and the theoretical loss will indicate coax that has suffered from a wet dielectric. Well installed coax connectors have essentially zero loss. The coax losses above are for 156.8 MHz, in the marine band.