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  Attentuation in UHF Series Connectors at VHF Marine Band Frequencies

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Author Topic:   Attentuation in UHF Series Connectors at VHF Marine Band Frequencies
chopbuster posted 08-05-2008 03:48 PM ET (US)   Profile for chopbuster  
To what degree or percentile would one expect to cause a VHF signal loss as a result of an in-line splice using marine grade PL-259 [and SO-239] connectors.

Would the mis-matching of RG58/U and RG8/U [coaxial transmission line] also contribute to signal loss?

jimh posted 08-05-2008 05:45 PM ET (US)     Profile for jimh  Send Email to jimh     
Coaxial cables have a nominal characteristic impedance (or surge impedance). For the cables of interest here, RG-8/U and RG-58/U, the nominal characteristic impedance is 50-ohm. There is some variation among manufacturers, and you might find some cables rated at 52-ohm. However, for RF transmission of FM modulated signals there is no significant difference between 50-ohm and 52-ohm cable.

Coaxial cables are often sold as "RG-8/U-type" or as "RG-58/U-type" when they fail to meet the qualifications specified in MIL-C-17-D, the reference standard for coaxial cable construction and quality.

Signal propagation on a coaxial cable is not free from loss, and cables are rated for a particular attenuation per unit of distance (usually 100-feet). Attenuation varies with frequency and is usually given in decibels. Attenuation varies with standing waves and increases with higher standing wave ratios.

Use of UHF-series connectors and adaptors adds attenuation as well as introduces slight mismatches because the UHF series connectors are not constant-impedance connectors.

Attenuation becomes more pronounced in cable and connectors as frequency increases. At 150-MHz the effect of two connectors and a barrel adapter probably introduces perhaps 1-dB of attenuation. An increase in attenuation of 1-dB is not a huge loss, but in any radio system any loss reduces the effective radiated power and reduces receiver sensitivity.

David Pendleton posted 08-05-2008 11:59 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
Just out of curiousity, are you talking about a splice as in connecting two ends together or splicing in a T connector or something similar?
chopbuster posted 08-06-2008 12:04 PM ET (US)     Profile for chopbuster    
RG58/u end to RG8/u end, using UHF connectors.
jimh posted 08-06-2008 02:36 PM ET (US)     Profile for jimh  Send Email to jimh     
I found this reference

http://www.k1ttt.net/technote/connloss.html

which reports on measurement of connector loss through PL-259 and SO-239 connectors versus frequency. This data is more optimistic than my earlier estimate, giving the loss as a rather miniscule, -0.015 dB.

Even allowing for some really poor quality connectors and sloppy craftsmanship in assembly, it would be hard to imagine you could get more than ten times that much attenuation, and that is only -0.15 dB, which is hardly a significant loss in any communication system. As a comparison, if a radio produced 20-watts output, to reduce it by -0.15 dB would mean an output of 19.3 watts.

If the connectors were high quality and well assembled so that the measured attenuation of only -0.015 dB were obtained, then the effect on a 20-watt signal would be a decrease to 19.93 watts.

dB = 10 LOG (P1/P2) --definition of decibel for power

(P1/P2) = 10^(dB/10) --definition of logarithms

chopbuster posted 08-07-2008 02:01 PM ET (US)     Profile for chopbuster    
Correction;

It's a RG 58/U to RG 8/X splice.

jimh posted 08-07-2008 02:32 PM ET (US)     Profile for jimh  Send Email to jimh     
There really is no such thing as a RG-8/X; it is a made-up grade of cable. I think it came about by using RG-59/U outer conductor with a foam dielectric and a larger center conductor. Most of the cable I have see marked RG-8/X is poor quality. I would never use it myself. But it is sold as 50-ohm cable, and thus there should be no mismatch using it in a 50-ohm transmission line system.

Describe what you are trying to build and we could give you some recommendations.

chopbuster posted 08-07-2008 11:25 PM ET (US)     Profile for chopbuster    
Another perspective on RG-8/X

http://www.cvarc.org/tech/powergain.html

In adition; http://www.shakespeare-marine.com/antennas.asp?antenna=5400-XT

I intend to install this antenna, my question has been the advisability to either eliminate the factory installed RG-8/X all together by terminating it at the antenna base and continue to the VHF radio using my existing RG-58/U cable with UHF connectors or use the supplied RG-8/X only.

jimh posted 08-07-2008 11:47 PM ET (US)     Profile for jimh  Send Email to jimh     
If the antenna has an attached feed line with RG-8/X, I would certainly not try to splice it or adapt it to a smaller feedline. Although I am not a fan of RG-8/X, it is much better to NOT have a splice or connector in the transmission line, and there is no rational basis for changing from a larger transmission line to a smaller one. Don't fiddle around, run the RG-8/X transmission line in one continuous run from antenna to transmitter.

I am not a fan of RG-8/X because in general I don't like coaxial cables with foam dialectric material. Also, the outer jacket is typically not Type-II Non-contaminating vinyl. There is a reason that a superior cable like RG-213/U costs a lot more than cheap cable--it's better.

I'd rather have a military-grade cable like RG-58C/U (as used on the GAM ELECTRONICS SS-2 antenna) than a non-spec cable like RG-8/X. In the twenty feet or less that is typically used on a boat there won't be any significant difference in the attenuation in the smaller cable, and the higher quality will hold up better in the long run. But if you already have the new antenna with RG-8/X, just use it. Don't splice it.

chopbuster posted 08-08-2008 12:32 AM ET (US)     Profile for chopbuster    
Would that senario hold true if the existing cable were Mil. grade RG-58C/U ?
jimh posted 08-08-2008 09:15 AM ET (US)     Profile for jimh  Send Email to jimh     
Again, I would choose:

--new cable
--one-piece cable
--no splices, joints, connectors, adaptors, barrels, tape,

over the alternative you propose.

I am sure that the Shakespeare antenna is furnished with a proper transmission line attached. Unless there were extraordinary difficulties associated with feeding this transmission line to the transmitter, I would use it.

bill4tuna posted 09-25-2008 08:49 PM ET (US)     Profile for bill4tuna  Send Email to bill4tuna     
jimh: I made the stupid mistake of breaking the Rg58 c/u cable while trying to twist off the base. Very stupid. This is the original cable on a US Coast Guard boat. The boat is a has a hardtop and the cable is fed through the hardtop. It would be very difficult to thread new cable and I would still have to attach the connector at the transmitter. What do you suggest I do to clean up my mess.
jimh posted 09-25-2008 09:17 PM ET (US)     Profile for jimh  Send Email to jimh     
If you have enough slack in the coaxial cable, install two connectors. Use cable-end mini-UHF connectors. Put a female connector on the cable that goes to the transmitter. Put a male connector on the cable that goes to the antenna.

Other options: Use BNC-series connectors (better, but more expensive); Use regular UHF-series (easier to find, cheap, but bulky).

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