Why adding coax changes swr

[GnG8d]

This antenna mounted on a heavy steel chrom mirror bolted directly to windshiel steel frame post with 40 foot of steel chassis and framing around coach so I caint understand why it would not have enough ground to work correctly.

What is the body made of? Is it sheet aluminum or fiberglass?

 

[Moleculo]

RV's are a real PITA to get working well with radios....at least mine is. That chrome mirror may have some type of protective clear coating on it, although you did say that you got a good match with the 102" whip with just one single piece of feedline.

Can you take a picture of the antenna and the cab and post it? It might help explain things a bit.

 

[dudmuck]

the answer is no,....... because radios don't "see" VSWR, they are not in any way, shape or form affected by VSWR.

VSWR doesn't actually exist,... it's just a math ratio.

the ONLY thing a radio "sees" is impedance.

you really should brush up on Smith Charts and you will find out that the "VSWR circle" is really an elipse.

I think you are near the truth.
SWR is another way of expressing return loss, which is a decibel expression of percentage of power reflected back into the RF source.

Standing waves do actually exist. It happens when you have energy of oscillating waves being reflected between source and load because the load doesn't absorb it all and neither does the source absorb it all, so it bounces back & forth, setting up peaks and troughs of voltage and current. Because two waves going in opposite directions of same frequency are running into each other.

The end result in radio transmitter is that with these waves bouncing back & forth (with high SWR), some of the power is absorbed by the load, and some by the source, and coax inbetween. With good SWR, most of power is absorb by load.

But when you have poor SWR then the final transistor and output matching circuit is being asked to dissipate more power, often more than its rated for. And depending on phase angle of reflected power, you could have too high voltage in final transistor, or too much current.. depending on length of coax.

I think this doesnt matter too much in the case of fiberglass RV, where the problem is likely the coax shield acting as a ground-plane for antenna.

 

[W5LZ]

That reflected power never gets to those final. It is re-reflected back to the antenna. What 'kills' those finals is operating them into an incorrect load. Impedance!
- Doc

 

[Captain Kilowatt]

The antenna is probably not tuned to a perfect 50 ohms at the feedpoint. Since one property of coax cable is it's ability to act as an impedance transformer, the reason the SWR is changing when the coax length is changed is because the transformation ratio is changing as the coax length is changed.

 

[Captain Kilowatt]

The antenna is probably not tuned to a perfect 50 ohms at the feedpoint. Since one property of coax cable is it's ability to act as an impedance transformer, the reason the SWR is changing when the coax length is changed is because the transformation ratio is changing as the coax length is changed.

 

[Captain Kilowatt]

let me ask you a question,....... has any radio ever been damaged by a "high" VSWR????

ever?

the answer is no,...... . because radios don't "see" VSWR, they are not in any way, shape or form affected by VSWR.

VSWR doesn't actually exist,... it's just a math ratio.

the ONLY thing a radio "sees" is impedance.

Yes and no. Since VSWR is a measurement of the ratio of what the impedance really is compared to what the radio wants to see (50 ohms) it can be said that a high VSWR can cause harm to a transmitter. A high VSWR is an indicator of a mismatch which in turn can cause damage to a transmitter. I can't ever recall having a severe impedance mismatch yet have a low VSWR.

 

[dudmuck]

That reflected power never gets to those final. It is re-reflected back to the antenna. What 'kills' those finals is operating them into an incorrect load. Impedance!
- Doc

You're defeating your first statement with your last statement.
But you are correct that power reflected into source transmitter gets re-reflected back to the load. And back again from antenna to load, just like an echo chamber.
The only way reflected power wouldnt get back to transmitter is by using a circulator.

With impedance mismatch (high VSWR), some of the power is being dissipated by the coax, some if it being radiated by (the bad) antenna, some if it is absorbed by transmitter output matching circuit, some if it is absorbed by the final transistor(s).

Some of you who are better experts on VSWR should go to Standing wave ratio - Wikipedia, the free encyclopedia
and edit in your better explanation of VSWR.

 

[W5LZ]

'Expert'?
You've got to be kidding.
- 'Doc

 

[colt45]

Hers some picts of rv with antenna I hope they come thru.

[/IMG]

 

[BOOTY MONSTER]

id change 2 things . id have some slack coax under the 259 so it isnt so tightly turned back . dont want the center conductor to move . and id change the antenna to a 27 inch 10k .
should do a good job for ya though .

if ya ever wanna wake that 29 up some try a 2018 xtreme mic on it

 

[colt45]

Sorry for the big picts, this is my first pict posting, I really am thinking I want an rc12970n2 but I gotta get this antenna stuff fixed first, I think the coax is acting like a conterpoise ground and I just think there is not enough ground plane on this fiberglass body to work so how can one do power or ham with this problem? You can see the quick disconnect that works well for changing to that 102 whip quickly but still that damn little red light on the cobra 29 tells me the swr will hurt the radio if left like this. If I find a solution I plan to hide that coax running up the outside of post to be hidden behind trim. I confused that you can make a counterpoised coax antenna with 102 inches each side and that can work so why is this rig not enough ground??

 

[Robb]

If the coach is mostly fiberglass, I would just run a 5 ft Hustler/FireStick or Wilson/SilverLoad on the upper mirror mount.

If there is 9 ft distance between the mirrors; then I would consider running two of identical aforementioned antennas and a co-phase harness. Then you will be rolling like an 18 wheeler 'good buddy'.

But seriously though, that might be better than to run a too tall antenna on the roof of a motor home. You will have clearance issues if you go that route. One mirror mount or even both are the real options - IMO

Get the antenna system working w/o the linear amp at first. Sort out the antenna problem before you can sort out any linear problems that may also be present afterwards.

Just sayin . . .

 

[Moleculo]

I think the problem is that the mirror mount isn't giving a good enough connection for RF ground. I'd try running some wide ground strap from the mount to ground and I bet that will do the trick.

 

[colt45]

Thanks for the replies, I think the coax is acting as a ground some and when I add 3 or 6 ft of coax the swr goes to 3 to 1.
Can someone tell me if a inexpensive tuner would adjust the swr to make it more usable and better then going to a no ground plane poor performance?
And with a tuner would this antenna swr at 3 to 1 work and tune for a 10 meter radio to work along with the 11 meter.