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Old 08-10-2011, 10:08   #16
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Re: Virgin at Tuning Masthead VHF Antenna . . .

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Originally Posted by btrayfors View Post
SWR generally should be measured at the radio.........
Bill
While it is very rare that I find myself disagreeing with Bill, I go so far to say it should be measured both at the radio and at the antenna.
Yes, I know that it can be difficult to measure it at the antenna but if you can, then do so. The reason being is that a lossy coax between the radio and the antenna will "hide or mask" a poor antenna. That is, the reflected power caused by antenna mismatch is absorbed by the losses in the coax so the SWR taken only at the radio end is artificially low. In fact a really lossy coax (providing its impedance remains at 50 ohms) may have show reasonable SWR even if the antenna is not connected!

IMO the ideal commissioning tests should be:
Measure power out of radio (into dummy load).
Measure SWR at radio into coax / antenna.
Measure power out of distant end of coax (into dummy load)
Measure SWR at coax / antenna junction.
Talk with a distant station.
This tells you if the radio transmitter itself is OK, what mismatches may be the coax system, what the loss of the coax is, how good the antenna is and finally that the whole system is working correctly (from your microphone to the distant station and back through your receiver and finally your speaker).
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Old 08-10-2011, 10:26   #17
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Re: Virgin at Tuning Masthead VHF Antenna . . .

Hey, feel free to disagree with me anytime! Especially, with a well-reasoned and factually accurate response :-)

While I agree with everything you said, in the real world most folks don't have access either to "the base of the VHF antenna" -- way up there atop the mast -- or to a good antenna analyzer which would help to ascertain if the coax or its connectors are in any way compromised.

It's even a stretch for some folks to have a SWR meter on hand (although EVERYONE should have one, IMHO).

At VHF frequencies and with good coax, the SWR measurements anywhere along the coax (at the end, at the base of the mast, at the radio) would be pretty close to one another. If any one of these shows a big difference, then there's something wrong with the transmission line or its connectors.

IMHO, again, it's not worth it to risk playing with old or damaged coax. The good stuff isn't that expensive (about $1 a foot for top quality MARINE RG-213U coax), and it's a vital part of arguably the most important safety device on your boat...the marine VHF radio. Just be sure the PL-259 connectors are put on correctly, with no shorts or opens, and are protected with adhesive shrink wrap and good electrical tape or the sticky stuff made for such connections.

If you do it right, you won't have any problems for the next 20 years :-)

Bill
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Old 08-10-2011, 10:59   #18
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Re: Virgin at Tuning Masthead VHF Antenna . . .

When you do measure the SWR record it in your log book or some other safe place (I tend to put it on the coax with clear tape over it as well).

Then later when you are having problems you can measure the SWR again and compare the two values. A significant difference lets you know that you have a problem.

Kinks, pinches, water intrusion etc all change the impedence of the coax and change the SWR.

Now if only you had a small TDR (but no need to go there).

Regards, Ethan
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Old 08-10-2011, 14:31   #19
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Re: Virgin at Tuning Masthead VHF Antenna . . .

Wow! Thank you all for the sage advice.

I will check on the drip-loop, I know the rigger wrapped it in self-amalgamating tape and then put glue lined heatshrink over the lot as well, but will also check for the drip-loop (with binoculars!)

So in short, I will cut the cable, leave a bit spare, install the connectors and consider myself out of the virgin category!

Thanks again!
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Old 08-10-2011, 14:49   #20
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Re: Virgin at Tuning Masthead VHF Antenna . . .

I just re-read your post and you have less than 10 ft (3m) extra cable in total. Leave it alone, IMHO.
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Old 08-10-2011, 15:15   #21
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Re: Virgin at Tuning Masthead VHF Antenna . . .

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I just re-read your post and you have less than 10 ft (3m) extra cable in total. Leave it alone, IMHO.
I have about 6ft hanging out, but this will need to be coiled if I don't cut it.. is coiling OK?
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Old 08-10-2011, 16:30   #22
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Re: Virgin at Tuning Masthead VHF Antenna . . .

Coiling is fine, just don't coil it to tight. There should be a spec. for your cable that defines the max. bend. It will probably be around 4" radius or so, but just guessing without looking it up. Coil it about a 12" to 16" diameter and you are good to go.
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Old 08-10-2011, 17:59   #23
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Re: Virgin at Tuning Masthead VHF Antenna . . .

Quote:
Originally Posted by akio.kanemoto View Post
Wow! Thank you all for the sage advice.

I will check on the drip-loop, I know the rigger wrapped it in self-amalgamating tape and then put glue lined heatshrink over the lot as well, but will also check for the drip-loop (with binoculars!)

So in short, I will cut the cable, leave a bit spare, install the connectors and consider myself out of the virgin category!

Thanks again!
OK, I hadn't realized the antenna and coax was installed by the rigger hence my earlier post regarding whether it was suitable and so on .
While I am not suggesting you do this, it is a common practice to use a short piece of small diameter flexible coax cable like 2 or 3 feet of RG 58 connecting the radio to the main coax (RG 213) for the reasons of convenience. Of course this does introduce some extra loss (minor) and the potential for future faults but both drawbacks can be controlled by good workmanship.

Quote:
Originally Posted by DeepFrz View Post
Coiling is fine, just don't coil it to tight. There should be a spec. for your cable that defines the max. bend. It will probably be around 4" radius or so, but just guessing without looking it up. Coil it about a 12" to 16" diameter and you are good to go.
The minimum bend radius for RG 213 is 55.5 mm (about 2 & 1/8") but I don't know if this can be extrapolated to a minimum coil radius. Anyway I wouldn't want to coil 6' so tightly as there would be 5 or 6 turns.
DeepFrz is on the money, bigger radius the better if you go down the coiling path.

Edit: totally off topic but thanks to this thread, I have just hit 2,000 posts - maybe I should listen (read) more and speak (post) less!
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Old 12-10-2011, 04:38   #24
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Re: Tuning masthead VHF antenna virgin...

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The less cable the better. Make the SWR test so that you know the installation is fine. Then make a real life test with a boat at a distance.

b.

It doesn't tell you the installation is fine, just that the standing wave ratio is OK which may indicate a good installation (and it may not).
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Old 12-10-2011, 05:02   #25
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Re: Virgin at Tuning Masthead VHF Antenna . . .

More to the point. The quality of the swr meter & its suitability at VHF frequencies being tested. What works for HF may not work for VHF. Lesson 101 should include a note to get a suitable swr meter. Log the readings you get as a reference to which you can refer to in the future if needed.

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Old 12-10-2011, 07:44   #26
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Re: Tuning masthead VHF antenna virgin...

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It doesn't tell you the installation is fine, just that the standing wave ratio is OK which may indicate a good installation (and it may not).
I bet you may be correct. I bet this and being able to send / receive over a long distance are the ultimate test.

Or not?

BTW Can you expand on the point 'it may not'? When the SWR test is OK and the installation is not - which elements can be wrong?

THX

Re the cable length I bark back what I posted: sure, if the excess is not huge it is better to coil the little extra - just in case we may need it one day and as an extra drip-trap too.

b.
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Old 12-10-2011, 07:50   #27
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Re: Tuning masthead VHF antenna virgin...

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Originally Posted by barnakiel View Post
I bet you may be correct. I bet this and being able to send / receive over a long distance are the ultimate test.

Or not?

BTW Can you expand on the point 'it may not'? When the SWR test is OK and the installation is not - which elements can be wrong?

THX

Re the cable length I bark back what I posted: sure, if the excess is not huge it is better to coil the little extra - just in case we may need it one day and as an extra drip-trap too.

b.
An example I had on a boat I was fixing - the co-ax was full of water - all of the RF power went in heating the water, there was little reflection so VSWR was low. hardly anything was radiated by the antenna.
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Old 12-10-2011, 08:04   #28
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Re: Tuning masthead VHF antenna virgin...

Quote:
Originally Posted by st599 View Post
An example I had on a boat I was fixing - the co-ax was full of water - all of the RF power went in heating the water, there was little reflection so VSWR was low. hardly anything was radiated by the antenna.
OK and then it gets found by long range test.

Life full of such small glitches.

THX
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Old 16-12-2011, 13:07   #29
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Re: Virgin at Tuning Masthead VHF Antenna . . .

OK, this took a little longer than expected (sold the house, moved etc etc... going nuts)...

BUT I installed the PL259's etc... and it works.... but it transmits only - no receive!

I've sent an email to Simrad support, but not sure when/if they will reply - but here is the email - any thoughts from our CF experts?

********
I have recently purchased a Simrad RS87 system and Simrad WR20 Remote Kit. These are both on my Simnet network and the WR20 base Station is connected to Station 4 on the RS87.

I have two issues and I am not sure whether they are related, however - the symptoms are as follows:

Issue 1: RS87 can transmit perfectly and is able to be received by other VHF stations, however the RS87 is unable to receive anything. Squelch has been adjusted up and down and at SQ=0, there is only static, confirming that the speaker works, however there is no actual reception of a VHF transmission. At SQ=1, static completely disappears, and there is still no reception. Transmit works perfectly however.

The VHF antenna and cables are new and since transmission works perfectly, I am assuming that there is no issue with these.


Issue 2: The RS87 becomes unusable when the WR20 base station is not powered by the Simnet network. I.e. when the Simnet network is powered down and thus the WR20 Base is powered off, the RS87 displays something along the lines of "Station 4 is occupied". When the WR20 is disconnected from the "Station 4" connector on the RS87, the message disappears and the RS87 is usable again. Likewise, if WR20 is plugged into "Station 4" and Simnet is powered on, there is no error message.
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Old 14-02-2012, 00:15   #30
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Re: Virgin at Tuning Masthead VHF Antenna . . .

In case anybody remembers this thread... I'm still alive.

So in this time, the RS87 was sent back to Simrad, officially pronounced DOA, repaired and sent back.

It works now.. what a pain in the a$$!!!

Still have the WR20 problem though.. VHF doesn't work unless the N2K network is powered on... bugger!
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