Help with a VHF SWR meter

[Ian_Edwards]

Hi, I've just bought an SWR meter to test my Raymarine VHF and antenna. The meter says that the SWR is 1.38. Is this OK.
It would be great to know if I need to spend some time on the co-axial cable, connections and antenna when the mast is removed for the winter next week.
I'd also like to know where I can buy a 50 ohm dummy load suitable for a 25 watt marine band VHF, all the ones I've seen online seem to be very expensive. Or can I simply make one using a good quality, high wattage, 50 ohm resistor?

 

[lw395]

It's pretty good.
But only 0.77 watt?

A decent 25W 50ohm load will be expensive.
There are various radio ham methods for making them.
High power resistors tend to be a bit inductive.
The normal approach is to use a lot of resistors in parallel, something like 10 3W 500R resistors radially from the centre of a connector to the ground flange.

 

[Skylark]

A VSR reading of less than 1.5:1 is good.

Assuming that you inserted the meter between the transmitter and the antenna, you have done all that's needed. Why do you want a dummy load?

If you are determined to buy one, I'd suggest setting a search on eBay. They regularly appear and often at bargain prices.

 

[oldgit]

It's pretty good.
But only 0.77 watt?

bout right on low power ?

 

[superheat6k]

I have an old SWR meter used for checking CB radio aerials. Can anyone advise if the SWR meter suitability is a factor of its specific operating frequency band or would this be suitable to check my Marine VHF aerial too ?

 

[lw395]

I have an old SWR meter used for checking CB radio aerials. Can anyone advise if the SWR meter suitability is a factor of its specific operating frequency band or would this be suitable to check my Marine VHF aerial too ?

Likely to be completely useless around 160MHz if it's designed for 27MHz I'm afraid.

 

[Daverw]

Sorry not suitable, needs to be VHF range, you can get HF and VHF models but not cheap like old CB ones, check out radio ham supplier sites

 

[Daverw]

Hi, I've just bought an SWR meter to test my Raymarine VHF and antenna. The meter says that the SWR is 1.38. Is this OK.
It would be great to know if I need to spend some time on the co-axial cable, connections and antenna when the mast is removed for the winter next week.
I'd also like to know where I can buy a 50 ohm dummy load suitable for a 25 watt marine band VHF, all the ones I've seen online seem to be very expensive. Or can I simply make one using a good quality, high wattage, 50 ohm resistor?

Easy to make, but costs of high wattage high tolerance resistors make it expensive, you cannot use wire wound ones. Also don't forget they will get very warm as the power goes into heat. You can find low power loads relatively cheap from ham radio guys as they mostly use high power loads these days, mine take about 2kw and oil cooled.

 

[William_H]

The 1.38to 1 VSWR in itself looks good. You might try it on a few different channels as most antennae will favour top middle or bottom of band. Also remember that any cheap lossy cable to the antenna especially if very long will make even a bad antenna look good. The VSWR meter measures power up to the antenna compared to power returned from antenna due to mismatch. Now 10m of cheap coax at 160mhz can lose several db of power say one quarter of power before it gets to the antenna. But then what is returned is also lost by one quarter so even bad antenna can appear good/mediocre because of the losses. So really best checked at the base of the antenna. (not base of the able) . But nevertheless if radio seems to work OK on range that is what matters. olewill

 

[oldgit]

I have an old SWR meter used for checking CB radio aerials. Can anyone advise if the SWR meter suitability is a factor of its specific operating frequency band or would this be suitable to check my Marine VHF aerial too ?

Really does depend on the meter.
The really cheap little generic square black box jobbies were really only for CB 26-28 MHZ, but some of the better quality ones ie. "Zetagi" do give reasonable indication at Mid and High band VHF.
Do not get to hung up on a super low SWR. If its under 3:1 its probably OK, anyway,with most marine antennas, there is nothing you can do to adjust it anyway.
It works or it does not.
90% of radio problems are PL259 plug and or cable , 5% 12v supply grief ,05% radio just giving up the ghost and 4.5% finger trouble.

 

[cpedw]

Having suffered from slowly failing VHF aerials twice, still receiving OK and not aware that I'm unable to transmit more than a 100 yards, I decided to investigate SWR meters.

I found this on the web and decided to give it a try. Mostly very cheap components; the meters were the costly items.

It seems to work on my new VHF aerial, reporting SWR of 1.1. I'm a bit outside my comfort zone working with radios but I think this result is probably optimistic. But I reckon that when the aerial starts to fail, this meter will show a change and that will do for me.

I'd be interested to hear comments from radio experts whether this really could be a useful tool to check on the aerial from time to time.

N.B. Very important to turn the transmitter to 1 watt when using the meter. It can't cope with 25 watts!

Derek

 

[Daverw]

It's highly unlikely you have 1:1 mostly impossible, at 1 watt very difficult to get any really SWR measurement as losses are greater proportion.
As said don't get to hung up on SWR as the issues as said are normally plug at radio and deck plug / socket.

Checking system with SWR meter nowadays crude as antenna analysers measure it better and actually check without radio attached so actually check what you are aiming for, also they check the coax and frequency ranges, they are costly though about £240

 

[Ian_Edwards]

The reason I wanted a dummy load, is to check the output power of the Raymarine VHF. I suspect that it is low on power. It works ok on receive, in that I can hear the coast guard on all their channels when cruising the NW of Scotland. However, I very rarely transmit, the exception is calling the Caledonian or Crinan Canal on 74 or a marina, but even then I'm more likely to use a mobile phone.
I was also confused by the 0.77 Watts in the display. It might it be the reflected power? The instructions are in Chinese English and bare understandable.
The cable from the set to the base of the mast is thin, but the cable to the mast head is much thicker. The connector at the base of the mast is clean and dry. I'll check the connection at the mast head when the mast is down.
Would it be worth measuring the SWR at the base of the mast?
I realise that the resistors will get hot, but I only need to click the TX button to get a reading of the output power.
I like the idea of 10, 500 ohm resistors in parallel, arranged radially from a grounded base. It sounds like something I could make.
I'm not sure how I would test the SWR at the antenna, it's not possible with the mast up. But I could make up a short patch cable and test is close to the VHF, when the mast is down.
Thanks for the replies, was always very useful.

 

[Skylark]

I really wouldn't bother making a dummy loaf. Plenty on eBay for your stated need with costs varying from £10 to £30.

VSR meters generally do not measure power (accurately). They'll show you that there's more output from your high power setting than low, but that's pretty much it.

If you are getting low VSR between radio and coax there's not a lot of point in moving the meter upstream towards the aerial at each junction. You would normally do that if you had a high reading in order to chase down the culprit.

Where are you based?

 

[Daverw]

Testing with the mast down will change the SWR anyway

 

[Daverw]

You could get hold of your local amateur radio club, someone there would help and have the gear

 

[Ian_Edwards]

Currently in Largs Marina, boat being lifted for the winter at Fairlie Quay on Tuesday.