Tips on using the SWR Meter for VHF and SSB/HAM

[autumnbreeze27]

So I got my MFJ-874 and want to use it on my rig before/after I make some changes.

I hooked it up to the VHF, set it to SWR SET, turned the dial so the needle went to CAL, then measured SWR, FWD, and REV
1.7 REV
2.8 FWD
2.9 SWR

I'm going to replace our mast head antenna today and wanted to see how bad it was. It's bent and looks like it's been burned or something, we generally aren't able to transmit very well.

I'm having a hard time finding info on how to use it, like what the numbers are telling me. I know I want a lower SWR, but not sure what the REV number and FWD numbers are really good for.

Any tips? Haven't tried it on the SSB/HAM yet.

[CarstenWL]

... SWR of 2.9 'is bad' ...

Your readings of 1.7 REV and 2.8 FWD are meaningless because you used the wrong scale ;-)
You can use the lower scales (depending on the Range selector) to see how much power is 'outgoing' (FWD) and how much is returned (REV).

Regards,

Carsten

[autumnbreeze27]

Ok, I have two scales, one is to 20 the other is to 200, so since I have it set to 200 since it's 25W I'm guessing I use the 200W scale?
What do those numbers show me?

I hooked it up to the HAM/SSB, set it to HF, and tried to set the SWR... The needle jumped up and then went to 0 and won't budge. Radio was on 50W, did I somehow blow it up or is it defective? It was still set at 200W so I didn't think I could blow it up with 50W?

[autumnbreeze27]

Ok using the 200W scale it looks like:
32 fwd
12 rev

Below are pics of my crappy vhf antenna I'm replacing

[CarstenWL]

Ok using the 200W scale it looks like:
32 fwd
12 rev

From a 25W VHF transceiver ;-)? Did you press the VHF/UHF Button?

I hooked it up to the HAM/SSB, set it to HF, and tried to set the SWR... The needle jumped up and then went to 0 and won't budge. Radio was on 50W, did I somehow blow it up or is it defective? It was still set at 200W so I didn't think I could blow it up with 50W?

You can not measure while in SSB (USB/LSB) mode ... Put the radio to CW to get a steady tone or press the PEP button in SSB and 'whistle' into the microphone.

Carsten

[autumnbreeze27]

Here's a pic, am I reading the meter wrong? I figured each little tick between 0 and 50 was 10. There is a tick under the orange needle in the pic.

Radio is a Standard Horizon GX2150.
Battery Voltage on the LifePO4 is 13.4
It's set to VHF/UHF, 200W, FWD, and calibrated with SWR SET and keyed to the CAL line.

Thanks for the CW/whistle tip!

[CarstenWL]

... 30W on the 200W scale (if in the 200W range).

'Some cheaper SWR meters' are 'messed up' if the SWR gets high (like in your case) ... and you want to measure output. In this case (if you want to know the output of your HF/VHF radio) it is best to use a Dummy Load ('artificial load') on the antenna side of the meter. You will see no return power (and a SWR close to 1:1.x) and the indicated forward power should be pretty accurate.

Regards,

Carsten

[ka4wja]

Autumnbreeze,
Well, you ignored the learned advice to buy Daiwa ($99 - $139) and instead bought MFJ ($99)???

{You do know what most hams call MFJ...."Mighty Fine Junk"...}

Oh, well....



As to your question...
The answers should be in the manual....but with MFJ, ya' never know!
SO...

So, here 'ya go:

1) You must have a carrier (FM, CW, FSK/RTTY) in order to properly measure SWR with this meter....
So, once you are set to transmit a carrier, then proceed with the following...

2) Set meter function to "SWR Set", transmit carrier and while transmitting adjust the "SWR Set Knob" until the meter reads full-scale (where the red "CAL" mark is on the top right-hand side of the meter face).

3) Then stop transmitting, and set meter function "SWR", transmit the same carrier, and read your SWR using the top meter scale...(which has 1.5, 2, 3, 4, 5, 10, and infinity gradations)


Understand that SWR measurements at the transmitter end will always be better than your actual antenna SWR (due to feedline loss), at HF this is negligible, but at VHF freqs this can make your readings erroneous by as much as 50% - 100%....so, on VHF if your readings are above 2:1, you have a problem!!
(And, from the looks of your antenna....you have a problem without even hooking up the SWR meter!!


With the exception of what the manufacture might label the controls/switches/adjustments and where the actual SWR "scale" is printed on the meter face, the above procedure is the SAME for all single-needle SWR meters...
(and, now you know why most recommend using a dual / cross-needle SWR meter on-board....makes life a LOT easier!!)




I hope this helps...


BTW, we never did hear back from you regarding your "noise" on HF...we never heard what noise you had...

Fair winds..

John


P.S. Please take note that the 874's accuracy is not very good....so, do not be surprised if the actual power output readings are different than what you expect, or read on different meters....

[autumnbreeze27]

Originally Posted by ka4wja

Autumnbreeze,
Well, you ignored the learned advice to buy Daiwa ($99 - $139) and instead bought MFJ ($99)???

BTW, we never did hear back from you regarding your "noise" on HF...we never heard what noise you had...

I updated the other thread when I got back from being off the grid. BTW your post about the daiwa came way too late, the other junk part was already ordered before we left the dock. I checked the SWR on my HAM rig and it's 1.25 before I added the isolator. I haven't added the isolator yet. Speaking of junk, when I use the SWR meter on my HAM radio it jacks up my boat wifi (the ubiquiti mounted outside). I had to reset it to get it working again. No issues when I transmit without the swr meter connected.

Even if my meter is off a little, it won't really matter, I won't be comparing it to other meters, and it still gives me a point of reference. I'm going to try and replace the antenna and see what my SWR does, hopefully I won't have to replace that end as well, but with my luck, I won't hold my breath.

I sent the wife up there today... but I may have to go up tomorrow... I hate heights!

Is there any reason to look at FWD/REV power readings?

[ka4wja]

autumnbreeze,
1) The forward ("FWD") and reflected ("REV") power readings are actually the forward and reflected power....and these measurements are used by the meter (or a human) to calculate the standing wave ratio ("SWR")....

Are they important to you??
That's up to you to decide...
But, knowing how much power your radios are transmitting (the forward / "FWD" reading) is always a good thing to know...
(if you have SWR calculations being done in the meter, and are not familiar with what reflected power is / what it does / etc., then knowing this number is probably moot....but since you have a way to measure it, I recommend that you do measure it, and everything else, and keep a log of the readings....just normal SOP for most hams!)



2) No worries on the MFJ stuff....I use one of their CW keyers, since my REALLY nice AEA died from corrosion...but other than that, I keep away from MFJ...
And, now you see why!!

(and I wasn't trying to make it seem like you needed an accurate reading....just passing on some info...



3) As for your VHF antenna...
--- Replace it!!!

As for your VHF coax....
--- If it's old (>10 years on a boat), it's suspect...

--- If it's really old (> 20 years), replace it...

--- If you need to replace the connectors, PLEASE pay a professional to do it, or make damn sure you have the proper connector and crimp tool/die....'cause it is unlikely that you'd ever get a soldered-on connector done correctly at the top of the mast (I don't even do that...although I have a friend that does!)



4) I did see that you updated your other thread, with Winlink stations being busy and some saildocs request details, but never any further info about noise...




I hope this has helped..

fair winds..

John

[Wotname]

Re the forward power function - in conjunction with a 50 ohm dummy load, it can be helpful in determine losses in the transmission path.

EG, measure forward power immediately at the radio RF connector using the dummy load as the "antenna" and note the reading - should be around 25 watts depending on the accuracy of the MFJ. Also tells the radio Tx is good etc.
Now measure the power at the far end of the coax, again using the dummy load in lieu of the antenna. Note the difference between the two and this is the power lost in the coax.

Record as John suggests and you can note any further loses in the future

[trifan]

Originally Posted by autumnbreeze27

So I got my MFJ-874 and want to use it on my rig before/after I make some changes.

I hooked it up to the VHF, set it to SWR SET, turned the dial so the needle went to CAL, then measured SWR, FWD, and REV
1.7 REV
2.8 FWD
2.9 SWR

I'm going to replace our mast head antenna today and wanted to see how bad it was. It's bent and looks like it's been burned or something, we generally aren't able to transmit very well.

I'm having a hard time finding info on how to use it, like what the numbers are telling me. I know I want a lower SWR, but not sure what the REV number and FWD numbers are really good for.

Any tips? Haven't tried it on the SSB/HAM yet.

Your VHF antenna is toast, basically. Also, to get the best results (since the distance from radio to antenna is several wavelengths), measure the SWR at the antenna when you put the new one up. Adjust for minimum SWR. Then the SWR reading at the radio should be below 1.5. (Maximum forward, minimum reflected). If the SWR is not that low then the cabling is bad. (you already tuned the new antenna). The same lightning that took out the antenna might have fried the coax (assuming you had a strike which is a safe assumption on a mast-mounted VHF).

Checking the SSB/HF antenna system is a bit different since there is most likely an antenna tuner involved.

[trifan]

Actually, Wotname's method is better overall. Go with that.

[Brigadoon]

a VHF antenna at top of mast is vulnerable to lightning strike
Would it be better to mount on a non-conductive bracket on spreaders (still subject to induced currents) or mount on deck away from stays and forgo the additional miles gained with a masthead mount

[donradcliffe]

My MFJ eyeball says your old vhf antenna looks a bit short--might have broken off, which could explain the SWR reading. It should be 3 ft long.

You want a replacement like

SHAKESPEARE Mariner 4200 3' 3dB VHF Sailboat Antenna | West Marine

The 4200 one will work fine--if you want to be an antenna snob you can spend a lot more.