Toggle sidebar Toggle sidebar
  • You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.

LOW SWR LOW OUTPUT HIGH SWR HIGH OUTPUT

Stellasstillarat

Stellasstillarat

Active Member
Aug 14, 2014
488
59
38
61
I've a local with what I assume to be a solvable problem. I don't know the answer but I'm sure many of you do. Equipment: cobra 2000 (40 channels), a99 5' mast on top his roof (ranch house) a dosey (swr/output) and 50' of lmr 400 that I suggested he purchase. Here's his problem. On channel one his swr's 1.1:1 and dead key is 1 1/2 watts. As he goes up in channels his swr rises and his output also rises peaking at 3 watts and an swr of 1.4 on channel 40. I explained to him his swr at it's highest was exceptabel and didn't think he'd fry anything. I also told him I would ask if there's s fix for this and thought it might be a simply matter of adjusting the rings on his a99 but i wasn't sure. So I ask you what's causing this issue and what's the fix? Thanks
 

Stellasstillarat said:
... Here's his problem. On channel one his swr's 1.1:1 and dead key is 1 1/2 watts. As he goes up in channels his swr rises and his output also rises peaking at 3 watts and an swr of 1.4 on channel 40...
Click to expand...

the reason is ,

his antenna is too long.
or
RF is getting into the meter.


adjust the rings on the A99 before you do anything else
 
You should check SWR readings at a higher deadkey at least 10 watts or more but shouldn't need no more than 25 watts for more accurate results.

Too low a deadkey won't give reasonably accurate results since the reflected power at a low deadkey can't be shown which is a result of a less than 1:1 match.

Your actual power output is the subtraction of reflected power to the forward power which decreases when a less than ideal SWR match is shown.

The antenna should have good bandwidth on 11 meters which I presume this will be used on. Better to set the SWR for the lowest possible match on the frequency that will be used most, then the antennas bandwidth will cover the rest of the band quite well..

That's old CB shop wisdom to set an antenna by matching channel 1 to 40. The people who insist on doing that haven't learned about antenna resonance points.
 
To give an example about tuning an antenna to it's resonant point which is usually but not always the lowest SWR reading, I pasted a pic of my antenna analyzer showing the bandwidth of a mobile antenna I was testing. The pic is the antenna out of the box before tuning and measured with a RIGmaster AA54 analyzer.

Notice at this frequency of 26.320, the graph shows this antenna is tuned to it's nearly lowest SWR point, then you can see if you were to tune a radio away from this frequency, you would be getting away from this point which is as narrow as 20 Hz.

So this antenna shown here can safely operate as low as 25.800 MHz and up to 26.800 MHz and be below a 2.0 SWR.while be centered and resonant at 26.320MHz.

This is how you should tune an antenna to use it's maximum bandwidth potential not balancing out channel 1 and 40.
img_20150316_141035-1-jpg.15412
 
DainBramage said:
adjust the rings on the A99 before you do anything else
Click to expand...

DO NOT DO THIS. Put a RF choke at the antenna feedpoint. The rings on A99 and Imax 2000s almost never need altering.

Part of the reason the power is going up is common mode RFI. Choking the coax at the antenna will stop that.
 
  • Like
Reactions: wavrider
DainBramage said:
the OP and his buddy are both complete newbies,......and he stated that,...............

do you think he understood anything you posted?
Click to expand...

Maybe, because I think I explained what should be done when checking SWR values on a antenna system simply enough.

"if you can't explain it in simply, then you don't really understand it well enough"
Albert Einstein.[/QUOTE]
 
fourstringburn said:
Maybe, because I think I explained what should be done when checking SWR values on a antenna system simply enough.

"if you can't explain it in simply, then you don't really understand it well enough" Albert Einstein.
Click to expand...
[/QUOTE]

ok then, .....could please 'splain to me how you can have a VSWR ratio of less than 1:1?:D

fourstringburn said:
... won't give reasonably accurate results since the reflected power at a low deadkey can't be shown which is a result of a less than 1:1 match...
Click to expand...
 

ok then, .....could please 'splain to me how you can have a VSWR ratio of less than 1:1?:D[/QUOTE]


Yes you can have a less than 1:1 match because 1:0 is perfectly flat or didn't you read that on ham universe.com?

Here is a pic which does show a mobile antenna with a near perfect match with a SWR below 1:1. This doesn't mean the antenna doesn't have other losses which it does in the coil inductance used to match the 50 ohm feedline since the antenna is shorter than a 1/4 wavelength for the given frequency.

What it is showing the resistive impedance (R) is equal to the magnitude of impedance (Z) giving a reactance value (X) of less than 0 in this case.

This shows that the antenna is of good design in regards to the coil loading to match a 50 ohm load ( really 52 ohms) and I have mounted the antenna with a good and solid D.C. ground.

Since this antenna is nearly 7 ft. tall, less inductance coiling is used to match it to 50 ohms therefore less losses from the loading coil which results in a better ERP ( effective radiating power) when compared against a 4 or 5 ft. coil loaded mobile antenna.

My previous Hustler RM series I replaced with the same mount on the same vehicle didn't give near as good as a reading as this antenna did.

For those interested, this analyzer reading is from a Sirio 5000 performer series 3/8 mobile mirror mount antenna.


img_20150316_140510-1-jpg.15414
 
Last edited:


Write your reply...
dxChat
Help Users
  • No one is chatting at the moment.


      You do not have the permission to use the chat.


      Top Bottom