Question methods of lowering SWR

[roadrage]

Can anyone describe, and then explain the benefits and drawbacks of using, for the purpose of matching impedence at the feedpoint of an antenna, the following devices:

Shunt coil/inductor vs. door knob capacitor vs. air variable capacitor.

I built and installed a shunt coil already but I was told that I would be better using one of the latter devices. I would like better understanding of them and their useage.

 

[Robb]

IIRC, the nominal impedance for a 1/4 wave steel whip is 75 ohms. Which is a 1.5:1 SWR. It is resonant at that impedance at that length - even though the impedance is less than a more desirable 1.1:1.

When using a 1/4 wave freestanding antenna, the way that the impedance is optimally lowered to a 1.1:1 SWR is to add 1/4 wavelength radials and set them at a 45 degree down angle.

 

[W5LZ]

A typical 1/4 wave is about 25 - 35 ohms (still in the neighborhood of 1.5:1 SWR), so the idea is to raise the input impedance, not lower it.
A coil is less efficient than a capacitor, but, there really isn't that much difference. The biggest reason for using a coil rather than a capacitor is convenience. It's a loteasier to wind that coil than to find the right size capacitor (variable or not). Also, when using a capacitor you should start out with just slightly more than the actual 1/4 wave length. That makes the antenna slightly inductive and adding that capacitance will bring it down to where it needs to be.
There's more to it than that, but that will get you in the ballpark.
- 'Doc

 

[roadrage]

A typical 1/4 wave is about 25 - 35 ohms (still in the neighborhood of 1.5:1 SWR), so the idea is to raise the input impedance, not lower it.
A coil is less efficient than a capacitor, but, there really isn't that much difference. The biggest reason for using a coil rather than a capacitor is convenience. It's a loteasier to wind that coil than to find the right size capacitor (variable or not). Also, when using a capacitor you should start out with just slightly more than the actual 1/4 wave length. That makes the antenna slightly inductive and adding that capacitance will bring it down to where it needs to be.
There's more to it than that, but that will get you in the ballpark.
- 'Doc

So how do I choose the correct capacitor to use? Or is a door knob capacitor a door knob capacitor? I am going to take a wild guess and say that it is a matter of installling one and if it is not the desired result, change out for a different value until its is.
I am on very shakey ground. I about half understand what I am getting into here.

 

[roadrage]

When using a 1/4 wave freestanding antenna, the way that the impedance is optimally lowered to a 1.1:1 SWR is to add 1/4 wavelength radials and set them at a 45 degree down angle.

I wish I could but its a mobile set up. My truck would look pretty funny riding around with a few 102" radials dangling down and whipping pedestrians on the kneecaps. :laugh:

 

[W5LZ]

That's how I'd have to find the right capacitor, try them till one worked. Unless you plan to run a LOT of power a 'doorknob' cap just isn't necessary. A ceramic disc with a hundred volt rating ought'a do it. Of course, if you happen to have several 'doorknobs' laying around...
- 'Doc


(Someone (MFJ?) makes a capacitive coupler box, several caps in it, rotary selector. That'd be one way. All things considered, that coil is just a handier way of going about it, cheaper, and there's really not much difference.)

 

[Beetle]

This is where a good antenna analyzer would pay for itself. You need to know the feedpoint impedance and whether the reactive portion is + (inductive) or - (capacitive). Then you'd need to calculate the amount of inductance or capacitance to give the same amount of reactance but with the opposite sign so the net impedance is purely resistive. For a mobile installation, the ground plane you have is what you have to work with.

 

[KM3F]

Why not get any copy of the ARRL antenna handbook that discribes all this?
No matter the year of the book, the theroy never changes.

 

[roadrage]

Why not get any copy of the ARRL antenna handbook that discribes all this?
No matter the year of the book, the theroy never changes.

I have one on my Ebay watchlist. As soon as I can spare the dough. Also, my wife said she doesn't mind if I sell my electric guitar and amp to buy an antenna analyzer. Being that I haven't used the guitar and amp for about 2 years, it won't bother me much to see it go. I just need to list them.

 

[The DB]

I have one on my Ebay watchlist. As soon as I can spare the dough. Also, my wife said she doesn't mind if I sell my electric guitar and amp to buy an antenna analyzer. Being that I haven't used the guitar and amp for about 2 years, it won't bother me much to see it go. I just need to list them.

Any idea which analyzer you are interested in?


The DB

 

[W5LZ]

That text about doing this sort of thing is a very good idea.
About a variable capacitor for doing that impedance matching. The biggest problem with them is mechanical rather than electrical. Where you gonna put it so that it's out of the weather, not easily bumped or misadjusted, get dirt/etc, between the plates? And then, depending on how much capacitance is needed, they can get huge very quickly. If you can handle all of that they certainly do work and are handy for retuning. They are more handy on fixed antennas than mobile ones for obvious reasons.
- 'Doc

 

[roadrage]

Any idea which analyzer you are interested in?


The DB

Probably just an MFJ 259 or something similar. It may not be the most accurate thing but right now I have a $20 SWR meter that just aint doing it. If I had only had a 4W radio with a tune and a mag mount, that SWR meter would be fine. I am getting in over my head with not enough equipment. Sucks that this hobby costs so much money to do it right. Seems like every upgrade needs 3 other upgrades to keep it all together. With my radio that swings up to 150W and a 102" antenna, I can't even out talk my ears when talking to my wife. And she is running my old Galaxy 929 (18W peak) and a K30. I can hear her before she hears me. Last night we did a radio check at about 9 miles. She could tell I was talking but couldn't understand me at all. She was coming in with some static but very readable. Something ain't right.

 

[Robb]

A couple of points:

There are a couple of things you can do for communication with your wife from her car to your base 1) get your base antenna higher 2) the K30 isn't what I call 'optimal' 3) try using upper or lower side band/SSB mode instead of AM.

AS far as analyzers go; I have not tried the Autek RF1 Antenna Analyst RF1 RF ANALYST. Autek Research . But at ~$140 new or ??? used on eBay, it might be a cheaper alternative. I have one of their 'true peak' reading watt meters and like it. Maybe if you could find a used RF1 Analyst for less than $100 - might be the way to go. It provides inductive and capacitive reactance values; those are essential to antenna tuning. Just a thought.

Of course, you might find an antenna that is not a basket case for $50 and save even more money. I know you are strapped; so there are alternatives too. You could always build a dipole or a loop antenna for pennies on the dollar for what you would spend for a new antenna and still get out like gangbusters. Just need a simple SWR meter and not an analyzer. Spending money is an option; not a necessity. The knowledge of building a loop or dipole has saved many others from spending cubic pounds of money to accomplish the same end. Hams have been doing it for years.

For local communications (35 miles or less), the antenna's "height - is might" axiom is one to remember. A dipole or loop is cheap and can be made from scrap. Don't spend money if you don't have to; and you really don't have to - IMO...

 

[roadrage]

A couple of points:

There are a couple of things you can do for communication with your wife from her car to your base 1) get your base antenna higher 2) the K30 isn't what I call 'optimal' 3) try using upper or lower side band/SSB mode instead of AM.

Of course, you might find an antenna that is not a basket case for $50 and save even more money. I know you are strapped; so there are alternatives too. You could always build a dipole or a loop antenna for pennies on the dollar for what you would spend for a new antenna and still get out like gangbusters. Just need a simple SWR meter and not an analyzer. Spending money is an option; not a necessity. The knowledge of building a loop or dipole has saved many others from spending cubic pounds of money to accomplish the same end. Hams have been doing it for years.

My radio and antenna that I am refering is my mobile set up on my truck. My base isn't up and running yet. I just rebuilt an old .64 wave aluminum base antenna. I tested it the other day but it isn't going up until I can get a tripod and mast, and some decent coax. It'll probably not be until after Christmas.


She won't put anything bigger on than that K30. I tried to offer her my old Wilson 1K but she won't take it. Still, it seems to be working ok for her. 9-10 miles on a K30 isn't too bad at all. It isn't even flat terrain. I am suprised I even talked her into letting me put the CB in her car. She only uses it to talk to me anyway. I once tried to get her to let me put a Magnum 257hp in her car. She thought it was "pretty", but she wouldn't let me get it for her. I think SSB would be too much for her anyway. She has no concept radios or any interest in them. She can turn it off and on, change channels, and key the mic.

 

[The DB]

You with a 1/4 wave whip can hear a k-30 transmitting 18 watts before they can hear you at 150 watts? Something is definitely wrong with that picture.

The 1/4 wave should clearly outperform the k-30, and that is before you factor in you are transmitting something in the neighborhood of 800% more than the other radio. They should hear you long before you hear them.

It does sound like the 1/4 wave is receiving well though, which suggests it is at least close to tuned properly. I am not so sure the antenna is your problem here. With few exceptions, an antenna should have the same gain for receive and transmit. The 1/4 wave is definitely not one of those exceptions.

On another note, the MFJ 259b antenna analyzer has its flaws, but it does work well. It is not a high end analyzer by any stretch of the imagination, but do you need a high end analyzer for 3 or 4 antennas? It is easy to use and the startup screen gives you the info you want. Also the most prolithic and innovative antenna builder on this forum uses one.


The DB