Galaxy DX88HL

Why restrict an antenna tuners to hams. They work for BCers just as well.

leon
kc0iv

 

No need for single frequency use

 

Will that "coax length" thing EVER go away? Man, that myth lives and BREATHES on CB radio and will NEVER die as long as there's a CBer alive! I have posed this question here many, many times. And I can PROVE it for the myth it is! YES! You CAN dicker with coax to make your radio "believe" that the antenna is "tuned". In fact, you can take a 50 ohm resistor, put it across the antenna jack, and it will read PURRRRR-FECT. And one of the reasons truckers believe the nonsense is, the bodies of trucks are non-metallic, and you can use coax at "certain" lengths that will cause resonant common mode currents @ 11 Meters, and the radio will "work"---to a point. In reality, it is NOT the way to achieve a real match that presents the best match to the radio AND the best signal out. And because voo doo "technicians" have gotten this result, they believe that CB radios MUST have 18 feet of coax to work. And I'm gonna show here how and why it is BULL###!
First of all, radio theory is the same for 27 MHZ as it is for 1.8 MHZ, or 118 MHZ, or 155 MHZ. So IF the "coax length" thing was really true, then it would be true for EVERY frequency from DC to Daylight. In fact, antennas can work WITHOUT any coax at all. OR.....................work ANY and all frequencies from 1.8 (that's ONE POINT EIGHT) MHZ thru 30 MHZ with ONE coax of no particular length, and do so with an SWR (it's NOT "SWR'ZZZZZZZZZ" but S W R) of 1.1 to 1 thru every single frequency within its range. Look on Google for "Screwdriver HF Antennas".
These are High frequency HF antennas for the Amateur and Commercial/Military bands. They basically have a movable loading coil inside a lower mast. A 12V motor extends the coil up or down to "tune" the antenna to frequency. Simply press a button in the cab/car to set the antenna to the chosen frequency. (Yes, they will cover 27 MHZ).

As an example, I operate both the military AND amateur bands with this one HF screwdriver style antenna on frequencies from 3.7 MHZ to 50 MHZ. OK. Now. IF the "coax length"thing was true, and since I work about 10 different HF and military bands, I would have to have 10 coaxes cut to a specific length for EACH band. One at 60 feet, another at 30 feet, another at around 20, 12, 8, and so on. Yet, I can use ONE antenna, resonant on EACH and ANY frequency I choose at 1.1 SWR, work the whole world, and (here's the kicker)......................the coax in my little truck is about 7 FEET LONG! The "formula" was.....well, the distance from the bumper to the radio & I didn't even measure it!!!

So what's the trick with trucks? Generally? TUNE THE ANTENNA, NOT THE COAX!!! Yeah, lots of trucks have plastic bodies, so you're gonna need to find some kind of "ground plane" to replace that lost metal that normally represents the other "half" of the antenna system. This is true for ANY radio antenna. Ergo, many so-called "radio techs" use coax to fool the radio into thinking its got something to load into. And coax is, I suppose, the quickest way, and the fastest buck! But it is NOT the right way to set up an antenna. But CBers have believed this bit of "radio myth" ever since the first CB sets were built in 1958. And part of it was, the antennas that were sold for CARS then had 18' of coax in the package. THAT just so happened to be the length needed to get from the bumper to the dash in those days, and CBers picked up on that as CB 'gospel'!
If you have a "normal" installation with a metal vehicle, coax length is simply a bunch of CB hooey, and I can quickly prove it with a demonstration of a screwdriver-style antenna!
GF

 

90° cables transform impedance
180° cables mirror impedance
Scientific fact not myth. Build any phased arrays lately?


Per ARRL antenna handbook.
(Read it lately)
A half- wavelength of any impedance transmission line faithfully
reproduces the impedance of the load (minus losses). With a 50 ohm antenna and no losses in the transmission line, the following is true.
Transmission line SWR Impedance seen at the transmitter

1/2 WL of 50 ohm 1:1 50 ohms
1/2 WL of 75 ohm 1.5:1 50 ohms
1/2 WL of 300 ohm 6:1 50 ohms
1/2 WL of 450 ohm 9:1 50 ohms

So you see a half-wave section matches the impedance of the antenna regardless of the magnitude of the SWR. If the SWR is anything other than 1:1, a half-wavelength (or multiple)
is the *only* length that will match the 50 ohms of the antenna.
180° of ladder line looks like :
Swr 9:1
R=50
Perfect match
Per the ARRL Antenna Book, 17th ed., p. 24-12:
"When the line length is an even multiple of 90° (that
is, a multiple of ? wavelength), the input resistance
is equal to the load resistance, regardless of the
line Zo... It does not matter whether the impedence
...is resistive, reactive, or a combination of both."

 

People are free to use any cable length they wish.
Anything other than 180° multiples will show a different input impedance than the load impedance hence your need for an antenna "tuner" and my lack of need for one.

 

Impedence does not change with the length of the coax, hence the swr won't change either.

perceived or real.

What you are quoting from the antenna handbook has some validity but doesn't apply here. We are not talking about phasing antennas nor are we talking about adjusting it for a delay line.

The change in the phase of the signal is done several ways, but it had little to do with a single piece of coax unless it is defective. You have use an open feedline and get the same results.

A good experiment is to use a fixed load (what the coax is designed for) at the end of any length of coax, apply a signal at 1watt and see where the swr is at.

If it is good coax without defects, it will be perfect.

It will always be prefect.

See one thing is recommending an antenna analyser, the other is learning how to use it and when under what conditions. You can get spurious reading from them if you don't look at the environment that they are being used in.

 

I think even you have to admit this only applies with a purely resistive 50 ohm load and any length of 50 ohm cable.
If the feed Point impedance is 45 ohms I think even you would have to admit the only way to present 45 ohms to the transmitter is to use 180° of cable or use a tuner.
It's actually misleading to tell somebody cable length means nothing if they have a feedpoint impedance of 45 ohms.
You forget to mention to them your theories only apply in a perfect world with a purely resistive feed Point impedance of 50 ohms.

 

You could opt for a no ground antenna but don't expect any miracles