Tuning jumpers and coax length

[W5LZ]

CWM,
In reverse order.

It comes up a lot (or mostly) on CB forums because of a lack of understanding. There's no requirement for knowing anything to use CB, as opposed to ham or other services. So, there's just no incentive to learn all the 'stuff' that would lead to understanding what's going on. As in, "If I don't have to, then why should I?". Until you decide that you want to know the 'why' behind what you do. Just like any other hobby/topic/idea, learning the 'why' isn't exactly easy and most people try to make do with a small amount of knowledge when that just ain't gonna work. At least, that's what I figure.

Why does feed line length make so much difference, or is soooo important? Cuz it's a 'cheap-n-dirty' way around really fixing the problem. Is that going to be a 'popular' answer? I don't think I'd bet much on it - LOL! Unfortunately, it's a matter of knowing what you're talking about, NOT accepting the quick and 'easy' answer. (That's not going to be all that 'popular' an idea either.) Oh well. If you want to 'fix' it by adjusting feed line length, be my guest. It'll keep you occupied any way. If you really want to know what the 'problem' is and learn how to fix it 'right', then please be my guest! There's no quick and easy way to learn it, takes effort, lots of thinking and the willingness to admit that you may not have had the 'right' answer to start with. In other words, a real P.I.T.A.!
All of that is a personal opinion. Don't agree? Okay...
- 'Doc

 

[Chuck Bravo]

W5LZ,
I installed my antenna and hooked it up with a random length of feedline. The SWR is right off the scale. What is the real problem here and how do I fix it. I would like to understand what's going on and fix it right. I am willing to think about it and I admit I don't have the right answer. Please let me know what the real problem is and most importantly, how do I fix it properly?

Tnx,
Chuck

 

[pinejuice]

please tell us more where did you mount your antenna?what kind of vehicle you have it on.the antenna could be the culprit?

 

[Chuck Bravo]

Hi Pinejuice,
It's a 102" whip on a 20 foot mast in my backyard. I can't add radials and I don't want to do a "cheap 'n dirty" coax fix.

W5LZ has made reference to a lack of understanding of what's going on. I don't want to make do with my small amount of knowledge so I am asking W5LZ to please enlighten me as to the proper method of fixing my problem. I was going construct a matching stub because I thought it is a simple impedance mis-match, but after reading the posts here, I now understand it's wrong to "fiddle" with my coax.

 

[freecell]

if you don't want to add radials you can fix it by fiddling with the coax. strip the jacket off of the coax immediately at the antenna on down for about a 1/4 of a wavelength and dress the shield down over the outside of the jacket to form a coaxial dipole. without the radial system you have only half an antenna in the air and swr can be 3:1 or higher without it. there are other options available if you are able to measure the actual input impedance and the amount of reactance present at the feedpoint.

using two dissimilar feedlines with regard to characteristic impedance in a series matching network will also remedy the problem. you could also try simply connecting a 1/4 wave element to the shield of the feedline at the antenna feedpoint and dressing it down the support underneath creating a simple dipole, especially if the support structure is non-conductive. if the mast you mention is a conductor it also could be incorporated as the missing half of a dipole. you make the comment that the swr is "off the scale" but this is not typical with a 1/4 wave element with the radial system of counterpoise missing. in the case of the 102" whip that you mention the vertical element may also be a few inches short of the length required. you might try using the dreaded "formula" to determine the length for a 1/4 wave radiator to figure the approximate length for your vertical radiator. if you did you would see that you're about 6" short.

246 / 27.185 = 9.049' or 108.5 inches.

it would be much easier to simply put a "whole" antenna in the air. your refusal to construct a radial or counterpoise system is creating the difficulty you are experiencing in part, for whatever your reasons.

you may also want to double check your feedline and make sure you have center-conductor continuity from one end to the other and that there are no shorts existing between the center conductor and the shield.

so your vertical element is too short and you are missing the other half of the antenna because you don't want any pesky radials and you wonder why you're having problems.

"understanding" would essentially constitute the ability to come up with at least two solutions to your problem. there's enough information above to keep you busy for most of the day.

 

[King Mudduck]

That steel whip 20 foot in the air is going to be one hell of a lighting rod!You do know that if you shop around you can get an antron for under $50.00 dont you?

 

[Cheech]

You can get an antron under 40 new! Thats a deal and it will work a hell of alot better!

 

[W5LZ]

Chuck,
Start with the fact that there are no 'one-sided' antennas, theres always the 'other-half' no matter what form it takes. That 'other-half' can be another radiating element the same length as the first (dipole/doublet), a few radials (always at least one), a vehicle's body, or just plain dirt with conductors running through it. So, what's the braid of your feed line connected to? The pole? That'll work, by the way, but the feed point impedance isn't gonna be anything close to 50 ohms, probably, so would take a little more work to make it so. There are several ways of doing that. One of them is by the 'coax length' thingy. Another way is by adding an inductance or capacitance in series/parallel. Another is by using a 'stub' as you've suggested. The whole idea being that you want to turn whatever the feed point's impedance is, into 50 ohms with no reactance, or as little as possible.
Now, to confuse it a bit, there are 'one-sided' antennas but they are usually multiples of a half wave length long. A dipole for example, is usually fed in the middle, but can also be fed at one end. With the end fed dipole, instead of an input impedance in the neighborhood of 50-75 ohms, the end fed'd input impedance is usually on the order of around 2000 ohms. But, an impedance matching network will get that 2000 ohms down to a usable value around 50 ohms. Same thing will work for almost any length antenna, the impedance matching network 'thingy' I mean. May have to use different value components, but you can still do it. Or, you can use a properly found length of coax of the right impedance as the matching network (~IF~ a coax of the 'right' impedance can be found (don't bet the farm on it)).
So what ~exactly~ should you do in your particular situation? Beats me, I don't know enough about your particular set up and/or situation to say exactly. What would I try? I'd try using an impedance matching coil at the antenna's feed point, going to ground (the pole). What value inductance? Another one of those "Beat's me!" thingys, so I'd make it variable by using a jumper clip (center conductor of the feed line moved between the various coil turns) to find the 'right' tap point. How do you find the right tap-point? Up/down/up/down/up/down till you get it right. Since I'm not the one going up/down, it's an easy solution!
- 'Doc

PS - Sorry if I made it sound like you were 'stupid', it certainly wasn't my intent or desire. Learning anything means you always start at the 'bottom' and 'learning' your way up to the top floor (or stop at any 'floor' that you want to). It's taken me almost 40 years to get what I know about radio stuff. Unfortunately, I learn best by making mistakes, which is a really stupid way to learn. (Right?) Do I know it all? Yeah! Right! (add a $1 with that and you can get a stale donut almost anywhere...)

 

[Chuck Bravo]

Doc,
No offence taken. Thanks for the reply.
I understand I can go the coax or coil matching method, it seems the coax method is easiest, but which method will produce the best efficiency or have the least losses? Also, which matching method will produce the greatest bandwidth for the antenna?

 

[BOOTY MONSTER]

The poll questions are asking the wrong question and only serve to confuse a confusing subject more!

Let's make this really really simple:

FACTS:
1: SWR is a measurement between the FEEDLINE and the antenna at the FEEDPOINT of the antenna (not the radio, not the SWR meter, etc!)

2: If your antenna is 44' from your radio, at 27MHz your coax should be 44' long. Yes, its really that simple.

3: If your SWR "changes" with different lengths of coax, you have a problem at the FEEDPOINT of the antenna. Fix that FIRST!

4: Coax length MATTERS if you are making up PHASING LINES.

5: To see the ACTUAL SWR at the FEEDPOINT, without actually being at the feedpoint, requires electrical 1/2 wave lengths of feedline at a SPECIFIC frequency! (It won't be accurate at any other frequency other than the one the coax is cut to!)

6: The correct formula to determin the FULL WAVE LENGTH (in inches) of a SIGNAL in FREE SPACE is: 11802.8/MHz
(Example: 11802.8 / 27.205 = 433.8467 inches. Divided by 2 and you get 1/2 wave in FREE SPACE)

7: Once you know your SIGNAL FREE SPACE length, you can determin your ELECTRICAL length in coax my multiplying the velocity factor of your coax.
Example: For channel 20, a half wave is 216.9233 inches.
216.9233 x .85 = 184.3848 inches for LMR-400.
216.9233 x .66 = 143.1694 inches for RG-213.
216.9233 x .78 = 169.2002 inches for RG-8X (foam)
(In feet for LMR-400 its, 184.3848 / 12 = 13.3654 feet)

8: If the feedpoint is correct, and you are not phasing antennas, ANY length of coax will work just fine!

This formula has worked really well over the years for phasing high power AM transmitter towers/antennas around the world. Some FM transmitters as well.

Walter Maxwell wrote a fantastic book and was a good friend to many of us! I highly recommend you buy his book!

BUMP

 

[hookedon6]

AAAARRGGGHHHHH!!!!

i just read this entire "wizzing contest" thread,............ and then i looked at the dates

it doesn't matter, the questions/answers are atill the same.

bottom line: for multi freq useage:tune the antenna for best VSWR @ the feedpoint, run coax to the rig, add a little for slack, and cut it off.

 

[Beetle]

What's a "tunning jumper"?

 

[rabbiporkchop]

coax

For best results use 180 degree phase angle or any multiple thereof. Otherwise your coaxial will act as an impedance transformer.

 

[6.0 Liter]

So..........."checking the impedance of an antenna, at the feedpoint, is never easy." I believe that statement has been quoted many times. My question is what is the easiest way to accomplish this using an analyzer, such as the MFJ 259B? How about some ideas on how some of you guys do that. Is a very short piece of coax used, or are there other ways to conect the device that are more effective?

 

[wavrider]

To many variables to give an answer directly.

Basically it depends on what type of antenna you are doing the testing on.

Vertical, Yagi, Dipole, Inverted V etc etc etc

If you want to "eliminate" the coax reading, use a 1:1 current balun if it is a balanced antenna, if it is an unbalanced antenna use an RF choke .

This should effectively, if constructed correctly, take the CMC off of the shield of the coax and allow the analyzer to just look at the antenna impedance.

If it is one of those type of verticals that require the coax as the other half of the antenna then using a choke may sort of screw that up.

Next thing to think about is, are you going to hook your transceiver to the antenna at the feed point? Or in that shack?

The antenna is just a part of the whole antenna system, still need to check the coax/feedline or whatever form of transmission line you prefer to use, lets not forget any type of grounding requirements whether it is Rf or DC ground, they all influence the complete antenna system.

Installing an antenna is more than just checking it with an analyzer, heck that is the easy part, installing the complete antenna system and make it work efficiently, now that is the accomplishment.