FAQ: What about the Coax?


Great question! We have had many questions asking about what type coax to use. In the Pre-Assembly Instructions of the Interceptor 10K Antenna I have suggested to NOT install a high performance antenna using cheap, old, unknown, or spliced coax. It is simply a Dime holding up a Dollar. Not good. The purpose of the coax as a transmission line is to transfer the RF Energy from one end to another end with minimum loss.


A few customers have called back to complain about the Interceptor 10K Antenna not working correctly. What we find is the old crappy, 1968 date coded, cheap, water soaked, spliced, unknown, too long & small, did I say cheap coax that doesn't have soldered PL-259 Connectors installed on it, causing the antenna or any antenna to not work, or work poorly.


There is no reason when installing a new antenna to not use new coax. The coax design and construction has significantly improved since 1968, even since 2000.


Case in Point:

A customer installed an Interceptor 10K Antenna. He calls me back to report that he can't hear anyone. We go over the I-10K Antennas construction and element lengths. All is good. I ask for a few photos of the antenna installation. I see in the photos old RG58C Coax with a date code of 1968 stamped on the Coax.


I send the customer a new length of Times Microwave LMR 400 Coax with the PL-259 Connectors soldered on the Coax. The I-10K Antenna is now working correctly, and he is making many contacts. This case in point has happened many times, so please be advised don't let cheap old coax degrade your antennas performance.


Here is a Coax specification and comparison chart from RFParts.com



Other Coax Specification Charts can be found on the internet, through a search engine.


You will see that LMR 400 Coax will have .7 dB loss at 30 MHz, with a length of 100 Feet. All Coax manufactures rate their coaxes loss in 100 foot lengths.


That means if 10 watts is sent to the 100 Feet of LMR 400 coax, that 8.5 watts will reach the other end at 30 MHz. This loss is called “Insertion Loss”. See Figures 1 and 2 below.




Figures 1 & 2 show how a 100 foot length of coax can be measured to figure the Insertion Loss of the Coax.


Figure 1 shows a radio at 30 MHz transmitting a output power of 10 watts, into a 50 Ohm Dummy Load.


Figure 2 shows the output power measured thru the 100 Feet of LMR400 coax. The lower power measured is 8.5 watts. This equals .7dB of Insertion Loss, which is ok per the factory specifications for the LMR 400 coax. All types of coax will have insertion loss. The chart shows the loss in dB for a certain frequency, and all coaxes loss specifications are for 100 Foot lengths.


This is how to figure the insertion loss, and a neat chart showing some dB relationships you should know.


dB = P1 / P2 Log x 10


Example: Looking at Figures 1 and 2


10 / 8.5 = 1.176


1.176 Log = .07


.07 x 10 = .7 dB


.7 dB Insertion Loss thru a 100 feet of LMR400 coax at 30 MHz.


Loss from 10 watts in, and 8.5 watts out the other end of the coax is .7 dB


So the loss of 1.5 watts or 15 percent equals .7 dB insertion loss, when the input power is 10 watts. This measurement verifies that the 100 feet of LMR400 coax is working properly. It is transferring the RF energy form one end to the other with its acceptable insertion loss. Be advised that the insertion loss dB’s here are going to be ballpark close. Not all the bands are shown in chart 1, but you should be able to get a understanding of the coax loss here. Another thing to know is that if a 50 foot length of coax is used the dB loss figure will not be exactly half, but close.


Chart 2 Showing dB’s in percentages of power loss.


.1 dB = 2.2 % Power Loss

.2 dB = 4.5 % Power Loss

.3 dB = 6.6 % Power Loss

.4 db = 8.7 % Power Loss

.5 dB = 10.8 % Power Loss

.6 dB = 12.9 % Power Loss

.7 dB = 15 % Power Loss

.8 dB = 16.8 % Power Loss

.9 dB = 18.7 % Power Loss


1 dB = 20 % Power Loss

2 dB = 36 % Power Loss

3 dB = 50 % Power Loss

4 dB = 60 % Power Loss

5 dB = 68 % Power Loss

6 dB = 75 % Power Loss

8 dB = 84 % Power Loss

9 dB = 87.5 % Power Loss

10 dB = 90 % Power Loss


The dB can be figured into a percentage by:


dB / 10 Inv Log /1 -1 = percentage.


The dB loss in coax has to be figured and considered so that you know what the manufactures specification loss is, and how close your coax loss is. In many antenna installations it s ok to have 3 dB or more loss in the coax, as the coax length may have too be very long and this is the only option the user has available.


A 3 dB loss will only be a ½ S Unit loss.


And there is no reason a station needs to upgrade from LMR 400 to LMR 900 for a short distance of say, 50 feet with a Amateur Station having a full legal output of 1500 watts. Considering all the numbers and keeping them in perspective is a good call.


For more info on the dB, and how to figure the dB from different units, see the FAQ at the web pages home page. Further additions are on the way.


Jay in the Mojave