what is your vswr at the antenna?

[Eldorado828]

Ok, so going into some antenna experiments at the house and going to out the rigexpert to work.

I hear you seasoned techs talking about swr at the feed point of the antenna.

If using per se rg8x as the jumper to that feed point, how long a jumper should a guy use?

 

[Crawdad]

I believe it's dependent on your frequency of operation. A half wavelength times the velocity factor of your 8x. The real experts will be along to correct me I hope.

 

[The DB]

Ok, so going into some antenna experiments at the house and going to out the rigexpert to work.

I hear you seasoned techs talking about swr at the feed point of the antenna.

If using per se rg8x as the jumper to that feed point, how long a jumper should a guy use?

The first question is, what version RigExpert do you have. The AA-55 Zoom and up you don't need to worry about coax length as it is possible to calibrate out the effects of the coax, there are instructions for this in the manual, look in the table of contents for the word "Calibration". If, however, you have the AA-35 Zoom, unfortunately there is no way to do this.

If you look at the AA-35 Zoom manual, on page 17, there are instructions on making a half wavelength coax for a specific frequency. A word of note, this half wavelength coax is good for exactly one frequency and one frequency only. It does not cover the entire range of the graphs that the device displays, or even the CB band. The further you get away from its tuned frequency, the less accurate it will be.

Honestly, because of this limitation, I highly recommend staying away from the RigExpert AA-35 Zoom, and getting the next version up. Yea, its about $50 more, but take it from someone who has worked with multiple devices over the years from the very cheap to the very expensive, the ability to calibrate out the effects of the feed line is worth much more than this small difference in price. Yes, it adds an extra step, but at the same time it also gets you much more accurate results that you can rely on.


The DB

 

[Eldorado828]

The first question is, what version RigExpert do you have. The AA-55 Zoom and up you don't need to worry about coax length as it is possible to calibrate out the effects of the coax, there are instructions for this in the manual, look in the table of contents for the word "Calibration". If, however, you have the AA-35 Zoom, unfortunately there is no way to do this.

If you look at the AA-35 Zoom manual, on page 17, there are instructions on making a half wavelength coax for a specific frequency. A word of note, this half wavelength coax is good for exactly one frequency and one frequency only. It does not cover the entire range of the graphs that the device displays, or even the CB band. The further you get away from its tuned frequency, the less accurate it will be.

Honestly, because of this limitation, I highly recommend staying away from the RigExpert AA-35 Zoom, and getting the next version up. Yea, its about $50 more, but take it from someone who has worked with multiple devices over the years from the very cheap to the very expensive, the ability to calibrate out the effects of the feed line is worth much more than this small difference in price. Yes, it adds an extra step, but at the same time it also gets you much more accurate results that you can rely on.


The DB

Thank you for the reply.
I have the aa-54 (not the zoom) and it appears to be as you're stating with this older model too.
So now I'll do some more reading and determine what I'll need.

Your help is greatly appreciated sir.

 

[bologna_sammiges4me@yahoo]

My old radio station back in the Appalachian mountains had 1000 feet of black plastic covered twin lead same stuff they used with tv antennas way back when ran up a big long mountain side all the way to the top with a ham international antenna in a tree..
When I got the rg8 coax coils just right on both ends my vswr was 1.0 needle never moved.
Well needless to say back in those days it wasn't nothing to talk skip allover the world on the daily.
This is irrelevant to the actual discussion I just wanted to throw my chicken into the fighting arena to watch it die.

 

[snippits75]

Just wondering why Avanti Specialists would recommend in their instruction manuals to use 12 foot multiples of sold dielectric coax(RG213), and 14 foot multiples of foam coax(RG8) if it did not matter?

Snip of a Moonraker 4 manual.

 

[The DB]

Just wondering why Avanti Specialists would recommend in their instruction manuals to use 12 foot multiples of sold dielectric coax(RG213), and 14 foot multiples of foam coax(RG8) if it did not matter?

So you think that just because a company put it out that it is absolutely true and must not be questioned?

When studying the Astroplane patent, an antenna patent that was held by the very same company, several of us have demonstrated several things mentioned in said patent were objectively false.

If they are going to make mistakes in a patent of all things, why would they also not make mistakes elsewhere?


The DB

 

[snippits75]

To stay on topic. I am talking about why they would recommend lengths of coax that are half wave multiples. Not about some mistake in a patent.

And, no, I don't believe every thing I read.

 

[Handy Andy]

To stay on topic. I am talking about why they would recommend lengths of coax that are half wave multiples.

Well, as I walk into any CB shop still around, most of the antenna and coax bundles and sets or "kits" always seem to have those very same measurements - length of coax and of course the antenna and it's whip being a "set length".

When you go back in time and compare - see how even the whip used in todays dummy-loaded base load antennas - like Lil wil and even K40 - you'll see how even the whip length has become shorter and shorter until nearly all the antennas are of uniform size and their coax also - uniform lengths and similar type (UG58X Mini-8 - or equivalent)

So when you look at the older peg boarded antenna kits, you needed room to display it, nowadays - its harder to tell the "fakes" from an original. They are all the same.

To me Avanti, Barjan, Diesel - or any new moniker name replacing said brand that used to be there - announced as; is now this...moment - that no matter what - we're ducks in believing their hype, in that; using this product will make you (or your stuff) longer stronger braver louder farther - just plain better - er, I'm just wondering if I need to call that 1-800 number on the package to make sure that their doctors are NOT playing with prescriptions' again...

 

[brandon7861]

To stay on topic. I am talking about why they would recommend lengths of coax that are half wave multiples. Not about some mistake in a patent.

And, no, I don't believe every thing I read.

Because at every half wave multiple, the input inpedance of the line is equal to what is connected to the other end. Although the R and X values will change as coax length changes, its absolute magnitude, ie. the sqrt(R^2+X^2), will always be the same (not counting losses). But only when the coax is a half wave multiple, the R and the X are the same as whats at the other end.

Think about it like a right triangle and the pythagoreas theorum (a^2 + b^2 = c^2). Infinite combinations of adjacent and opposite sides can make the same hypotenuse, likewise, an infinite combination of R and X values can make the same SWR or reflection coefficient. By having the coax at a half wave multiple, you are not only measuring the hypotenuse (the absolute magnitude of R and X, aka the reflection coefficient, or using a little math from that, the SWR), you are also getting the correct side lengths (the true R and the X at the antenna). This makes designing matching networks a breeze with a smith chart. Transmission lines (discounting losses) are just impedance transformers that vary R and X while keeping their absolute magnitude equal, and every 180°, it repeats.

 

[brandon7861]

I'll put it a different way just for fun.

Lets say you had an antenna that was not 50Ω. You then connect a 50Ω coax to it, then in the shack you use a tuner to match the other end to 50Ω. If you dd that with a half wave multiple coax, you could copy that matching network exactly and install the same one with the same values between the coax and antenna (and remove the tuner at the radio) and get the same effect.

 

[brandon7861]

For people with analyzers like the old rigexpert (DB mentioned this), they cannot account for X° of coax. With a nanoVNA or newer rigexpert, you can either calibrate right to the end of the coax or port extension to that plane and get accurate measurements. The older rigexperts and MFJ analyzers just read what was at IT'S port, not what is X° down the line. For that reason, they recommend a half wave coax.

Edit: why does it always take me three posts to say what I gotta say??????? LOL

 

[brandon7861]

imagine you and a buddy just erected your 1-10k antenna up the hill behind the house,
you have 500ft of rg213 running from the shack to your antenna,

you have no or insignificant common mode current on the braid and the plugs are fitted correctly,

your buddy sits in the tree & hooks his vswr meter right at the antenna feed-point,
he always measures at the feed-point, he won't have none of that coax stuff messing with his readings,

you run back to the shack & hook up your identical vswr meter to the rig with a double male connector, tune to 27.205mhz and key the mic,

1.2:1 says your meter in the shack,

what vswr does your buddy's meter read ?

I'll give the 10 year old question a shot while I am here.

500' of RG-213, assuming 1.4dB/100', has 7dB loss. Lets say the radio in the shack is a 100w radio (for sake of discussion). For there to be a SWR of 1.2, 8w of that 100w must be coming back in as a reflection. What would that 8w need to have started with 500' away before it went through 7dB of loss? 40w! The next question is how much of that 100w made it down that 7dB of coax loss to the antenna to be reflected. 20w did. So you see, you didn't measure 1.2 in the shack, that would be impossible. You have no chance at all of measuring the SWR through 500' of RG-213! The antenna simply cannot reflect twice what it receives! Sorry this answer came so late. The answer is infinity x2 lol!

Edit: I was totally off by one decimal point, see post #168

 

[Shadetree Mechanic]

Now I have something to think about.

 

[Riverman]

LOL, Chris. That’s exactly how my head feels.

My solution is to continue using coax in lengths of 50’, 75’ or 100’ (depending on which best fits my setup) and measure SWR in the shack. Makes it a whole lot easier to sleep at night.

Good or bad, right or wrong, I don’t know, have never known and don’t want to know what my SWR is at the antenna.