RG-213 connected to RG-58 cable noobie cb question  - Page 1
Forum section: electronics / beginners

author: rickselectricalprojects
author_rank: Regular Contributor
timestamp: April 30, 2015, 12:22:32 pm
content: Hi everyone!I want to improve my radio reception buy purchasing some lower loss cable and i was wondering if it id possible to cut most of the RG-58 cable off the antenna (leaving about 30cm. It comes with 5m of cable) and connecting it to about 8m of RG-213 cable. I would connect them using PL-259 and SO-239 connectors. Purchasing a different antenna is not a option. there aren't any antennas that you can change the cable and i also can't afford another antenna since i am only 14 would there be any extra loss? Would that short bit of RG-58 make the RG-213 have the same loss as RG-58? Would the connectors cause extra loss?Thanks!

author: rickselectricalprojects
author_rank: Regular Contributor
timestamp: April 30, 2015, 12:24:12 pm
content: Also in australia we have 477 mhz cb so the frequency would be 477 mhz

author: cowana
author_rank: Frequent Contributor
timestamp: April 30, 2015, 12:40:33 pm
content: Cable loss is specified per meter, so the short bit of RG58 won't degrade the performance of the RG213 section.Some back-of-the-envelope calculations:Loss in 5 meters of RG58: 1.5dBLoss in 30cm of RG58: 0.1dBLoss in 4.7 meters of RG213: 0.7dBLoss in PL259/SO239 @477MHz: ~0.6dBSo in the original situation:5 meters of RG58 = 1.5dB lossIn your 'upgrade':30cm RG58PL259SO2394.7m RG213Total = 2dBSo, for a run of 5m it seems that the overall loss will be greater using the parts in your example. Obviously the longer the cable run, the greater the advantage of replacing the cable.PL259/SO239 connectors are reaching the limits of their abilities (regarding reasonable loss) once you approach 500MHz - you'd get much better results switching to N connectors at that frequency. The graphs below show the loss of two PL259 connections being >1dB, while the loss from two N connectors is barely noticeable:Source:http://www.qsl.net/vk3jeg/pl259tst.htmlAndy

author: rickselectricalprojects
author_rank: Regular Contributor
timestamp: April 30, 2015, 12:55:13 pm
content: Quote from: cowana on April 30, 2015, 12:40:33 pmThanks for your quick response, it was very informative and extremely helpful.I will also be using 8 meters of RG-213 and that is tons better than 8 meters of RG-58Thanks again

author: GreyWoolfe
author_rank: Unknown
timestamp: April 30, 2015, 01:01:59 pm
content: The connectors will cause some loss. Any time there is a break in the cable, there is some loss. Minimize this by proper soldering technique and proper waterproofing. Wrap the joint with self amalgamating tape starting from the bottom up if the cable is vertical. This gives you the waterproofing. Wrap quality electrical tape such at Scotch 33 or 88 in the same manner which will give UV protection. Also tape the coax to the mast below the joint with some slack to take the stress of of the connection. Cowana is right on, N connectors are the best to use for VHF/UHF operations. Not sure how PL-259s became the 'standard'. It is great to use N connectors at the antenna but you still have SO-239 at the radio unless you convert to N. Interesting that CB is 477 MHz there, here it is 27 MHz. I was in CB back in the 70's when you actually had to have a license. Many years later I became a ham radio operator and love the hobby.

author: w2aew
author_rank: Super Contributor
timestamp: April 30, 2015, 05:00:06 pm
content: Quote from: rickselectricalprojects on April 30, 2015, 12:22:32 pmIf you run the numbers using the information that cowana gave you, you'll see that you're not going to re-coup any loss. In fact, adding additional connectors, etc. means that you'll wind up adding more loss than you're saving with the RG-213. If your end goal is to improve reception, then even if you save a few dB of loss, this will be barely noticable. You've got to improve SNR or signal strength by several dB - usually 6 or more - before the improvement is somewhat noticable.IMHO - save your money for a better antenna, one with some gain, to get some noticable improvements. Or better yet, build yourself a little yagi antenna - pretty easy for 477MHz.

author: Flump
author_rank: Frequent Contributor
timestamp: April 30, 2015, 06:55:58 pm
content: you would be better off getting the antenna higher if you canat uhf even 5ft can make a big difference

author: retrolefty
author_rank: Super Contributor
timestamp: April 30, 2015, 07:40:56 pm
content: One thing is for sure RG-58 is certainly very poor performance at that frequency and length.

author: G0HZU
author_rank: Super Contributor
timestamp: April 30, 2015, 10:00:38 pm
content: Whilst I don't recommend keeping the RG213+ short bit of RG58 I would like to comment on the VK3JEG VNA plots and say they they look a bit odd. A properly set up two port VNA should not produce a noisy plot like that for something as simple as a few adaptors in series across LF to 500MHz.Also, I would expect that an N to PL to barrel to PL to N adaptor combo of reasonable quality would have maybe 0.3dB loss at 477MHz.You probably can buy something cheap and nasty that is made of cheesy metal that will be worse than this but 1.25dB loss seems very high to me.But at 477MHz I think it's a good idea to keep the transmission line simple as in keeping it as just one piece of RG213. I think the biggest problem with the adaptors and the RG58 will be keeping it reliable wrt connection quality (over time) and also keeping it weatherproof.

author: VK5RC
author_rank: Unknown
timestamp: April 30, 2015, 11:28:03 pm
content: +1 Re looking at your antenna, lots more gain to be achieved there in general, look at height, from which direction signals may be coming: these will help choose the 'best' design eg yagi, stacked 1/4 wave etc. ARRL Antenna Handbook a classic in this area and very practically based.

author: G0HZU
author_rank: Super Contributor
timestamp: May 01, 2015, 01:08:39 am
content: I don't understand why people are accepting and working with the VK3JEG data. Am I the only one who thinks the measurements are a bit odd?However, I searched on the web and there are other hams measuring similar loss for a similar setup and I'm left wondering if they are using the test equipment properly?I don't have the adaptors here to replicate the VK3JEG tests but a simple analysis based on the dimensions and physics of the adaptor setup would suggest to me that the VNA measurements are suspect. Especially the S22 measurement.Source:http://www.qsl.net/vk3jeg/pl259tst.htmlCan the adaptor combo really get that far around the smith chart at 500MHz? How long is the total adaptor combo in mm? I would query the VNA calibration procedure in all of those plots. There could also be something horrible about the way the VK3JEG N to UHF adaptors are constructed internally. i.e. they could be designed badly.PL259 connectors and the F-F barrels are horrible things but they shouldn't be 'that' bad at 477MHz. I don't use PL259 connectors for work or research here at home but I probably have a few old ones somewhere from my days on ham radio. Maybe I'll buy some adaptors and see what I can measure.But I think the 4.7m RG213 + 30cm RG58 is a bad idea from a reliability point of view even though I think the overall loss will be less than 5m of good quality RG58. This assumes that reasonable quality PL/SO UHF connectors are used.

author: G0HZU
author_rank: Super Contributor
timestamp: May 01, 2015, 09:51:55 pm
content: I found a few adaptors at work today and the first measurement was for this:A piece of RG213 about 13" long with an N type + N type F-F barrel at one end and the other end is a PL259 then a SO239 to N Male adaptor.So the whole thing becomes a very long N Female to N Male with some coax and an SO236/PL259 in the middle. This makes it easy to measure with a VNA.The measured insertion loss for the whole lot was just 0.24dB at 477MHz. Some of this loss is in the coax, some is in the N connectors and most is in the PL/SO connection.I also repeated the test using an Anritsu thermocouple power meter with very low sensor VSWR and I use an Agilent E4433 sig gen fed into a HP 10dB attenuator (rated to 12GHz) to get a good source match.After normalising with the test cable removed, the cable assy above measured 0.23dB loss at 477MHz.I did the same thing using an E4406A signal analyser with an extra external 12GHz 10dB attenuator on its input to minimise uncertainty. I used a 10dB 12GHz pad on the signal source and again got 0.23dB loss at 477MHz when the above cable assy was put in line.

author: G0HZU
author_rank: Super Contributor
timestamp: May 01, 2015, 10:03:25 pm
content: I next measured a 'BNC to PL259 adaptor' to 'cheapo CB quality SO 239 barrel' to 'PL259 to BNC adaptor' and measured 0.27dB loss for the whole lot in series at 477MHz.

author: VK5RC
author_rank: Unknown
timestamp: May 02, 2015, 03:21:41 am
content: Interesting info. I find pl259 quite hard to know if they are done up correctly, would you be able to try some measurements with the pl259 slightly loose?

author: G0HZU
author_rank: Super Contributor
timestamp: May 02, 2015, 12:12:58 pm
content: Quote from: VK5RC on May 02, 2015, 03:21:41 amit's hard to define slightly loose but I understand your concerns here... For this very reason I wouldn't recommend having a few extra connector joints in the coax run at 477MHz.But to give you some idea why I was sceptical of the plots (and conclusions) from VK3JEG I initially measured the dimensions of an SO239 connector on the back of an old CB radio.The inner has a diameter of about 5mm and the outer is about 9mm. But I would expect that a SO239 barrel would be closer to 10.5mm ID inside its length.The dielectric material will be something like PE with a dielectric constant of about 2.2. If the barrel is about 26mm long then you can predict the insertion loss using a simple RF simulator and a simple coax cable model. The barrel will act like a very short piece of low impedance coax (much lower Z than 50 ohm. Possibly 30 ohm?) and this will introduce an impedance bump.But the connector isn't very long wrt a wavelength at 477MHz so the impact won't be that great. A crude simulation predicts about 0.2dB (mostly reflective) loss in the barrel at 477MHz. The model also predicts that the return loss degrades to about 14dB by 477MHz.I can post up the simulation plots and the VNA plots of a real measurement if anyone wants to see it?The results agree very closely...

author: rickselectricalprojects
author_rank: Regular Contributor
timestamp: May 03, 2015, 11:18:11 am
content: Quote from: G0HZU on May 01, 2015, 09:51:55 pmWow thank you so much for putting in all that effort to help me out. I really appreciate it