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Author Topic: Mini-RG8x for VHF?  (Read 1534 times)
KD8UEG
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Posts: 6




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« on: March 20, 2013, 10:56:47 AM »

Hello,

I just received a new radio in the mail today and have an Arrow yagi antenna that will be here on Friday.  My problem is I don't have any cable to hook it all up!!!  I can't get any RG8 from Amazon by Friday but I can get RG-8X.  My question is, will the RG-8X work?  I have read a lot about feed line loss and was wondering how much I will experience and if using this smaller cable will damage my radio???  The only length I can get quickly from amazon is 100ft with PL-259 connectors already installed, is this too much cable?

73
Steven KD8UEG
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W5CPT
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Posts: 556




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« Reply #1 on: March 20, 2013, 11:28:43 AM »

Yes it will work BUT you will loose a lot of signal both RX and TX in the attenuation at VHF frequencies.  You did not say how long your cable run needs to be. I would not use 8X for a permanent installation of anything over 20 feet.  At 100 feet if you put 50 Watts into that cable, you will deliver to the antenna (assuming a perfect match) about 18 Watts.  And your incoming signal will suffer the same degradation.

You would be better served if you were to wait (I know you don't want to) and figure out how much cable you need and buy just that length and buy a cable that will not dissipate your signal as heat rather than RF.

Clint - W5CPT -
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WB6BYU
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Posts: 12974




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« Reply #2 on: March 20, 2013, 11:33:53 AM »

You can check the losses of various cable types using VK1OD's handy
Transmission Line Loss calculator here:

http://vk1od.net/calc/tl/tllc.php

For example, the loss on 2m in 100' of Belden 9258 (RG-8X) is 3.8dB, so
about 42% of your power reaches the antenna.  If a 50' piece will reach,
then the loss would be only 1.9dB, so 65% of your power is radiated.

Does it make a difference?  Depends how much margin you have on the
radio path.  Probably isn't detectable for local repeaters, while there may
be cases where it makes the difference between being able to pull your
signal out of the noise or not at the other end over marginal paths.

Summary:  RG-8X works, but not as well as RG-213 (though it is more
flexible and easier to work with.)  Longer lengths will have more losses
than shorter ones when the antenna is in a fixed location (but if the
longer length allows you to mount your antenna 30' higher in the air,
it is worth the added loss.)  It all comes down to signal strength:  when
signals are strong enough the loss doesn't matter.  On weak signals it
may.  There is no absolute answer.


One other point:  coax loss or the type of 50 ohm coax will NOT damage
your radio.  It simply affects the amount of power that is lost between
the radio and the antenna.  The only time that affects the radio is when
you have to run it on high power all the time and it can get rather warm.
It will do that regardless of the type of cable, of course, but sometimes
a good enough antenna system will allow you to run it more often on low
power.
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KA4POL
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Posts: 1853




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« Reply #3 on: March 20, 2013, 11:37:18 AM »

Well, somehow I can understand your excitement about getting into the air. But stay calm anyway.
You did not say how long the antenna cable should actually be.
The data for RG8X tell you a loss of 3.1 dB /100ft on 100 MHz and 4.5 on 200 MHz. So you can see this is not a big issue. The same is valid for the connectors. On VHF this is not a problem.
You did not say what radio you got. Staying below 200 W output will be fine with the RG8X cable.
Later on you will be able to do some of the soldering of connectors yourself. So you'll be able to cut the cable to the required length anyway.
Do you need//have a power supply  Wink
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KD8UEG
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Posts: 6




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« Reply #4 on: March 20, 2013, 11:42:29 AM »

I found some Andrew CNT-240 30ft and it should be long enough for me, looks like the loss with this cable would be much less...
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K5LXP
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« Reply #5 on: March 20, 2013, 11:42:41 AM »

was wondering ... if using this smaller cable will damage my radio???

Just the opposite.  Lots of lossy cable would present a better load to the radio.  The downside of that of course is it's not efficient.

Perfection is the enemy of good enough.  Put it up and use it.  The worst that can happen is you might need to replace it if that few dB of difference a premium cable has makes a difference with whom you're trying to communicate.  

I've got nothing but 8X on my VHF and UHF "local" FM stuff, probably upwards of 50 feet.  Even if I switched to hardline it would not make a difference in what I do with it.


Mark K5LXP
Albuquerque, NM
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KA4POL
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Posts: 1853




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« Reply #6 on: March 20, 2013, 11:53:52 AM »

I found some Andrew CNT-240 30ft and it should be long enough for me, looks like the loss with this cable would be much less...
According to http://www.commscope.com/catalog/andrew/product_details.aspx?id=27206 the loss is 3.1 dB/100ft at 150MHz. Not such a dramatic difference.
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AA4PB
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Posts: 12638




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« Reply #7 on: March 20, 2013, 12:19:44 PM »

30-feet of RG8X only has 1.3dB of matched loss at 146MHz. I wouldn't worry in the slightest about using it.
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G4IJE
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Posts: 236




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« Reply #8 on: March 20, 2013, 12:20:09 PM »

It's worth remembering that cable loss is reciprocal - you lose exactly the same "amount" of signal on receive as you do on transmit. Say you hear a station who is running 50 watts and they are S7 with you. If you call them with your rig set to 50 watts they should also hear you at S7. With lower loss cable you may hear the same station at S8, and then he should receive you at S8 also. As long as you can hear the stations you want to work, they will hear you too, assuming power levels are the same.
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VE3FMC
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« Reply #9 on: March 20, 2013, 02:16:15 PM »

Hello,

I just received a new radio in the mail today and have an Arrow yagi antenna that will be here on Friday.  My problem is I don't have any cable to hook it all up!!!  I can't get any RG8 from Amazon by Friday but I can get RG-8X.  My question is, will the RG-8X work?  I have read a lot about feed line loss and was wondering how much I will experience and if using this smaller cable will damage my radio???  The only length I can get quickly from amazon is 100ft with PL-259 connectors already installed, is this too much cable?

73
Steven KD8UEG

Hi Steve

I ran RG8X for years on a home brewed 5 element 2 meter quad. Mind you the run was only 38 feet in length. At the time I really didn't care about 2 meters so I was not going to spend money on better grade coax. For what I used that band for that run of coax did just fine. But I would not run much more than 50 feet of it, too much lose.
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K1CJS
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Posts: 5809




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« Reply #10 on: March 21, 2013, 06:45:56 AM »

I regularly ran a 2 meter FM rig at my shack, 50 watt output into a Hustler G7-144 omnidirectional gain collinear antenna with 50 feet of RG8X connecting the two.  I have no problem at all with contacts fifty and sixty miles away--sometimes even mobiles at that distance.  RG8X isn't going to make that big a difference in the real world unless you're doing weak signal work.  For an average station, RG8X for VHF will be fine.  
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W8JX
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Posts: 5324




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« Reply #11 on: March 21, 2013, 08:33:02 AM »

I regularly ran a 2 meter FM rig at my shack, 50 watt output into a Hustler G7-144 omnidirectional gain collinear antenna with 50 feet of RG8X connecting the two.  I have no problem at all with contacts fifty and sixty miles away--sometimes even mobiles at that distance.  RG8X isn't going to make that big a difference in the real world unless you're doing weak signal work.  For an average station, RG8X for VHF will be fine.  

Yes the extra 1.5 or so db loss over low loss cable is not the deal killer that many like to think it is.
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WA3SKN
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Posts: 5419




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« Reply #12 on: March 22, 2013, 05:58:12 AM »

How long a run, and what frequency and mode?
100 Ft of coax might be an issue for weak signal VHF/UHF CW/SSB work, but will be OK to use for any FM operation!  And MOST hams now start with FM and repeaters.  It is easier to run but does have foam and needs to be protected from water intrusion!

-Mike.
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K8AXW
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Posts: 3593




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« Reply #13 on: March 22, 2013, 09:39:18 AM »

Steven:  The cable loss has been covered quite well so now I'm wondering why you ordered it from Amazon?  There are plenty of cable suppliers that offer expedited shipping if you're willing to pay the shipping costs.

I use Amazon for quite a few purchases but always go to an outlet directly for my cable and connector requirements. 
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N6AJR
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Posts: 9879




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« Reply #14 on: March 22, 2013, 10:25:36 AM »

just use what you can get.  Most of the time you will be working a repeater and usually a couple of watts to the antenna will be enough for full quieting.  If you were doing moon bounce or long distance ssb or cw 2 m stuff I would reccommend  better cable but it will work just fine for a first time setup and save a bit of cash for your next toys.  I have several 50 foot runs of rg8x for 2m/440 stuff and never think twice about it.Its like your car. a caddy will do 100 mph and a VW will do 85 mph, but most of the time you only drive 65 so either will work just fine.
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