looking to add linear

Ok MsJamie,
I'm trying to learn here so not an argument from me.
I thought that the antenna was part of the overall electrical length of the "system". No?
Antenna physical/electrical length does not affect swr?
Induction due to arrangement of the coax doesn't change the resistance in the system?

I just don't understand everything I thought I knew I guess.

Like I said above...I want to learn from ya'll so please advise!

Thanks...928

edit...you & Handlebar seem pretty sharp on this stuff so would one/both of ya explain this to me please?
Fact 1. Length of the coax does change my swr & always has.
Fact 2. Length of the antenna does change my swr.
Fact 3. Orientation of my coax does change my swr.
Fact 4. When swr is low at the amp, it's happy, as is the radio.
Please explain what I'm missing here?
Why is there something wrong with my antenna when I routinely talk across the continent on 100 watts?

Hey Big Dog,

Were you over on the eastcoast around the carolinas,georgia, florida last week?
Do ya remember 928 outa wyoming talkin to ya? The name BIG DOG is ringing a bell.

 

There are at least two parts to an antenna system, the antenna and feedline.

The radio expects a 50Ω impedance.
The feedline (coax) is nominally 50Ω.
The antenna should be 50Ω. If it is, we have a 1:1 match. If not, which is usually the case, we have power being reflected back down the feedline, and then the feedline is acting as a matching network.

There's a lot of complex math involved (I clawed my way through it my senior year in college for my BSEE), but the short version is that with those "standing waves" (the SW in SWR), there are peaks and troughs in the impedance, and if you catch the right spot, you can get a 1:1 match. (At least until you change frequency...)

The physical length of the antenna (not including the feedline) certainly does affect the SWR. If the antenna is properly tuned and matched, the SWR will be at a minimum. Lengthening the antenna lowers the resonant frequency (and the point of minimum SWR); similarly, shortening the antenna will raise the frequency.

More about antennas than you ever wanted to know:
http://www.antenna-theory.com/

 

Ok....Lets look at the whole coax question another way....

Say your putting up a 75-100' tower in the back yard for a CB Antenna for a base set-up...

How are you going to tune that antenna using different lengths of coax, if you get your best SWR reading using only 60' of coax? Are you going to do like they did with the telephone on Green Acres and stick the radio half way up the tower?

You hook up the radio and antenna...then adjust the height of the antenna to get your best SWR...There is no magical length for the coax...

UNLESS, your trying to do a co-phase set-up using two antenna's or are trying to set-up two antenna's in a particular out-of-phase array at which time exact coax measurements are critical.

 

OK I'm trackin'
The guy that sold me the little RM had a big fancy meter that he hooked dirrectly to my antennas 259. He said the antenna wasn't resonating & swr was high.
I said OK,,,check it at the radio please. He got in the truck and plugged his meter into the coax at the radio and BAM...swr was 1.1. He couldn't explain that. I sure don't know why either. I chalked it up to "it's all good" as at the amp swr was near-perfect. We put the amp in-line eith a jumper, and swr was 1.1 at the radio as well.
As I said, theres ALOT about radio that I don't know but would like to. Not enough to go to school for tho as I own a truck. This info will have to do.
Thx again but I still don't get understand your point. What's wrong?

 

BTW Msjamie, I believe you are refering to viewing the swr (standing wave reflected) on an oscilloscope <sp?
I actually do understand the wave and the base line & their relationship. For dirt-clods like me, the "r" is most imporntant as it's the power (for lack of a better term) that's reflected back down the "feed line" to the finals in the radio/amp. Don't typical swr meters calculate this reflected power as a % age? (Lord I hope I ain't been this wrong for so many years! )

Again,,,resistence is a total of the circuit is it not? Antenna & coax are part of the circuit are they not? Arn't ohms for a circuit not measured end to end? Are swr meters anything but a glorified ohmeter? (maybe this is where I'm getting lost?)

Showing my ignorance again,,,please explain.

 

Twolane, hate to say this but I do alot of DXing myself and never heard you out there. I hit both coasts top to bottom most of the time with my little Connex 4600 turbo, RK 56 through a perfectly tuned Wilson 5000 on the top of a Peterbuilt.

 

I guess things change as you get older.When I first got involved with radio and DX'ing, it wasn't considered DX unless the contact was outside of the country....

 

Never heard of you either! Oh-well...

Yea gettin' old sucks for sure. Dosn't mean you or I are stupid tho.

When I started it was CQ,,,not DX and the radios were old tube Johnsons and Brownings. Guess times do change but "radio" and what works hasn't.


I'm 10-7,,,ya'll have a great night.

 

I believe the nodes repeat every half wavelength, or ~12 feet for CB on standard coax. The other option is to put a choke on the coax at one of these nodes; then the remaining coax isn't part of the tuning stub.

That is correct; that's the way it should be done.

 

You can't really display SWR on an oscilloscope...

SWR (technically, VSWR; voltage standing wave ratio) as our meters show, is derived by the ratio of forward to reflected power. It's not a direct correlation, though. And yes, some SWR meters do show % reflected on the scale.

We are dealing with impedance here, not resistance. Resistance is part of impedance; reactance (capacitive or inductive) is the other part.

Say you have a 1/4 wave vertical antenna over a good ground plane. If the antenna is at resonance (exactly 1/4 wave long), it would have a resistance of ~37&#937;, and a reactance of 0. (That would be written as 37+j0 &#937;.) It would read on a VSWR meter as 1.5:1.

If we then detune the antenna (either by changing the length or the frequency), the reactance would start to increase. If we get to X=37 (Z=37 +/- j37), then the shown impedance would be Z=sqrt(37^2 + 37^2) = ~52&#937;, which is pretty darn close to 50&#937;, and would barely move the needle on reflected power.

SWR bridges are not at all related to ohmmeters. Ohmmeters measure resistance, which is independent of frequency. SWR meters measure the ratio of forward to reflected power.

Take a look at this meter, called a "match needle" meter:



It shows both forward and reflected power at all times. If you look at where the needles cross, the red line shows what your SWR is. This should give you some idea on how the forward and reflected power relate to SWR.