SOTA – Coax cable signal loss at 432 MHz
Using good quality RG58AU MilSpec coax at 144 MHz is a compromise of weight saving versus RF losses and the ability to coil a 5 metre length of coax to fit in the SOTA back pack. Yes I hear you, considering the losses in RG58AU it’s not ideal for VHF/UHF weak signal reception!
Using RG58AU at 432 MHz was a temporary measure but in doing so my UHF set up will have losses in the coax feedline in the order of 33% or more. Assuming a perfect VSWR and no losses in RF connectors (unlikely), the cable loss will be 1.72 dB. The reality is my UHF set up is more like 3 dB loss where half of the FT-857D power output is lost in the coax cable before any amount of RF signal is successfully radiated by the antenna. It’s not all bad (yes it is) my 70cm Yagi has a theoretical gain of 10 dB. 😉
When correctly matched to a 70cm antenna with 50 ohm feedline the Yaesu FT-857D has a maximum power output of 20 watts. Assuming a perfect match between the transmitter output and the antenna, I am wasting at least 7 watts in a short 5 metre length of RG58AU. I know the antenna VSWR is not perfect and the connectors are PL259s so let’s go with a 3 dB loss, half of the radio’s power output. On receive, 3dB loss in the feedline/RF connectors is an issue I want to address.
Options for upgrade, remember Summits on the Air (SOTA) is a hobby not an obsession or an addiction (ha ha fits of laughter) 😉
- purchase a 70cm amplifier – No
- purchase a different Yagi antenna – No point, the 70cm 7el Yagi has a theoretical gain of 10dB
- replace my surviving coax with low loss coax at UHF frequencies, while keeping an eye on weight and cost. Yes!
Coax cable upgrade. After a few hours of research around amateur radio circles, blogs and the wider WWW on the properties of coax cables, dB loss per metre, bend-ability (my technical term for minimum radius) and weight, I decided LMR-400 or a variant would be a good investment. Of course the weight of any length of coax is a factor any fair dinkum SOTA activator would consider, LMR-400 has a weight of 100 grams per metre. For my SOTA 70cm and 2m antenna arrangements I am after two 6 metre lengths of LMR-400, each length fitted with N connectors at both ends.
Comparison of popular coax cable losses at 144 MHz (cable only)
|144.2 MHz||Length (m)||Power In (watts)||Cable Loss (dB)||Loss %||Power Out (watts)|
Comparison of popular coax cable losses at 432 MHz (cable only)
|432.2 MHz||Length (m)||Power In (watts)||Cable Loss (dB)||Loss %||Power Out (watts)|
A visit to a local Canberra coax cable supplier revealed the local market is supplied with Andrew CNT-400 coax cable manufactured in China by COMMSCOPE. CNT-400 has a published attenuation of 8.86 dB/100 metres at 450 MHz similar to LMR-400 at 8.9db/100 metres. Both cables have a weight of 100 grams per metre.
Photos: © Copyright 2016 Andrew VK1AD
I proceeded with the purchase of two 6 metre lengths of CNT-400.
Sunday, 1 May 16 I made plans to activate Isaacs Ridge VK1/AC-041 on 2m and 70cm. My aim was to deploy the new coax cable combined with a dual band 2m/70cm Yagi. Unfortunately the SOTA gods had different ideas turning on heavy rain and strong north-westerly winds. I will try again next weekend, possibly Saturday given the current Wx forecast for Sunday 8 May 16 is 70% chance of rain in the range of 1 to 6 mm.