impedance matching help

[BammBamm]

I have been reading about impedance matching a antenna and I am confused. I do not own a mfj antenna analyzer nor do I plan on buying one any time soon, that said is it possible to wrap my own coil to impedance match a 108' SS whip or do I have to have a analyzer? If I wrap 1" loops 7-9 times at about 1" long of a finished coil would I just then adjust my antenna for best SWR?

 

[The DB]

You don't have to match that whip to a feedline. If you do something is wrong.

How big of a groundplane is below the antenna?



The DB

 

[Beetle]

Is this attached to a vehicle or ?

Oh - and what frequencies do you plan to use this if other than CB?

 

[BammBamm]

As for whats the ground plane? My newest inhereted car a 1985 Buick Riviera with 62,000 original miles and it's loaded! Uncles old car as he decided it was time for a Caddy. I am thinking about a RCI 2970n2 and through reading on another post on here I got into the whole impedance matching info and wondered if it would benefit me at all. Right now my Magnum 257hp is in it with a Wilson 1000 magnetic as I can't decide where or how I want to mount my antenna.

 

[M0GVZ]

You only need to do impedance matching with seriously shortened antennas (think screwdriver antennas on 80m) using a coil with a high Q installed with a good ground plane or where you're using a non-resonant antenna and your tuner doesn't have a wide enough range.

The 108" whip is a quarter wave on 11m and has no coils so shouldn't need any matching.

 

[W5LZ]

You really don't know if you need to do any impedance matching until you mount that antenna and check the SWR. If the SWR isn't at least close to or under 1.5:1 then you may need to do some matching. One way of doing that is by using a coil between the center conductor and ground. What size coil? I can't answer that, it isn't going to be huge by any means though. Try a size and then either add to it or remove turns till the reading gets reasonable.
- 'Doc

 

[9C1Driver]

So far I don't see what the problem is. If you roof mount, trunk mount, or mag mount a quality 11m antenna you should have no problem setting the SWR. At that point you won't have to change a thing for 11m & some freeband talking.

 

[Grim Reaper]

Using a magnet mount isnt the best answer but,that wilson is a 5/8s wave electronically and the whip is a quater wave theorectically the wilson will out talk the whip.pattern radion angles,anyway no coils needed to match up just use rg 58 or mini 8 coax,if rg59 is used it will have a impedance issuse,

 

[The DB]

Using a magnet mount isnt the best answer but,that wilson is a 5/8s wave electronically and the whip is a quater wave theorectically the wilson will out talk the whip.pattern radion angles,anyway no coils needed to match up just use rg 58 or mini 8 coax,if rg59 is used it will have a impedance issuse,

The thing is they use that 5/8 wavelength long wire for loading the antenna as well, and a load on an antenna determines electrical length not by how much wire was used, but by the reactance that it adds. It also adds additional losses that a full length whip does not have.

It is a different thing entirely to say they used a 5/8 wavelength long wire to make an antenna and the antenna is an electrical 5/8 wavelength, unless of course you are in marketing...

Even if it really were an electrical 5/8 wavelength, you now have a matching problem as its impedance and reactance are no where near where they would need to be to have a low SWR...

Further, it is the physical length in this case that determines the radiation pattern, which means you will have the radiation pattern of a 5 foot antenna, which is not as good as the radiation pattern of a 8.5 to 9 foot antenna at these frequencies.


The DB

 

[W5LZ]

A typical 1/4 wave whip will have an input impedance somewhere around 35 ohms give or take a bit. That means that the best SWR you could get is something like a 1.5:1. That's not just on 11 meters but on any band. If you want to keep an antenna resonant (a biggy), then changing it's length to change SWR is shooting your self in the foot. There's no significant difference between a 1.5:1 and the ideal 1:1 SWR, just not worth messing with.
Laying a not 1:1 SWR off on the 'groundplane' is sort of silly. At HF there isn't goping to be 'enough' vehicle to make a decent 'other half' (the 'groundplane) of that whip antenna to start with. And the vehicle isn't the onl;y thing that's making up that 'other half' of the antenna! The dirt/concrete/whatever under that vehicle also plays a part in it. So how can you tell what the antrenna is actually doing as far as resonance and impedance matching goes? Sorry, you can't do that with only an SWR meter. That SWR meter can't tell you what you need to know to 'correct' things. It never has and it never will. It can certainly tell you a little about what your antenna is doing, but only if you know a lot about what impedance actually is and what SWR actually is. It ain't simple! If you use an SWR meter to tune an antenna you will probably have as good an antenna as anyone else who does it that way. That does not mean that it's as good as it can get. Nasty thought? Yep, but it's a fact anyway...
- 'Doc

 

[jazzsinger]

It can certainly tell you a little about what your antenna is doing, but only if you know a lot about what impedance actually is and what SWR actually is. It ain't simple!
- 'Doc

You've certainly proved in the past it ain't simple for you, its certainly not what you think it is, knowing what it is and understanding how it works are 2 completely different things.

 

[jazzsinger]

You only need to do impedance matching with seriously shortened antennas (think screwdriver antennas on 80m) using a coil with a high Q installed with a good ground plane or where you're using a non-resonant antenna and your tuner doesn't have a wide enough range.

The 108" whip is a quarter wave on 11m and has no coils so shouldn't need any matching.

Conor:

Assuming a steel whip in an air dielectric has a velocity factor around .97
then 108" is only a 1/4 wave around 26.5 Mhz,at 27 Mhz its closer to 106 ", at 27.18 5Mhz its around 105 inches at 27.500 MHZ its closer to 104 inches and at 28 Mhz its approx 102"


300/26.000/4 x.97 = 2.79m x 3.28 = 9.18 feet x 12 for inches = 110.13 inches at 26 Mhz.

300/26.500/4 x.97 = 2.74m x 3.28 = 9 feet x 12 = 108.05 inches at 26.5 Mhz.

300/27.000/4 x.97 = 2.69m x 3.28 = 8.83 feet x 12 = 106.05 inches at 27 Mhz

300/27.185/4 x.97 = 2.67m x 3.28 = 8.78 feet x 12 = 105.33 inches at 27.185 Mhz (US centre frequency)

300/27.500/4 x.97 = 2.64m x 3.28 = 8.68 feet x 12 = 104.12 inches at 27.500 Mhz

300/27.79125/4x.97 = 2.62m x 3.28 = 8.59 feet x 12 = 103.03 inches at 27,79125 Mhz, (UK centre frequency)).

300/28.000/4 x.97 = 2.59m x 3.28 = 8.52 feet x 12 = 102.26 inches at 28 Mhz

all figures rounded up or down as appropriate to 2 decimal points.



As you can clearly see, only one small section of 11m is actually 108" around 26.500 mhz. you will also notice for every 500 khz there is roughly a 2" jump.

Conor how can you say 108" whip is a 1/4 wave at 11m, thats only true for a small and nigh on unused part of the band around 26.500 Mhz. I'm sure you'll change the laws of physics again to prove me wrong.

for all you US guys buying 102" whips you'll also notice for your cb frequencies a 102" whip is too short, which is why you have to add springs to bring swr down unless you plan using it on 10m. get either 108 inch one and trim it, or even better a 108 " two piece one that can be adjusted at the centre for different lengths.



the mathematics is done for you, you gonny argue this one too?

Feel free to reeducate me,only tool i used was the calculator on my mobile phone, no online calculators or shite like that, feel free to double check my figures


NO BULLSHIT, NO SNAKE OIL, NO TRICKERY NO HYPE, JUST STRAIGHT FACTS AND FIGURES TO PROVE IT !!!

 

[jazzsinger]

Using a magnet mount isnt the best answer but,that wilson is a 5/8s wave electronically and the whip is a quater wave theorectically the wilson will out talk the whip.pattern radion angles,anyway no coils needed to match up just use rg 58 or mini 8 coax,if rg59 is used it will have a impedance issuse,

A Wilson won't touch a 1/4 wave whip, it can't even match the Sirio range, its got no chance against a full size whip.

as the DB has pointed out to you its not 5/8 wave, its a shortened, read less efficient 1/4 wave.and factually its a 1/4 wave with huge capacitive reactance, with an equally inductively reactant coil to cancel out reactance, the only thing theoretical is its a good antenna.

 

[M0GVZ]

Snip a load of waffle..

108" is a stock length. You cut it to tune it to the frequency you want.

:headbang

 

[Captain Kilowatt]

108" is a stock length. You cut it to tune it to the frequency you want.

:headbang

So is a 102 inch whip. At least on this side of the pond they are.