MFJ-207 Antenna Analyzer

[Alan Georges]

Continuing MFJ’s fine tradition of “good enough for the job and not one bit better, and at a fair price too,” this is their budget antenna analyzer.  The main points about it are that it works, but doesn’t have super resolution.  Pick a frequency band, twist the frequency tuning knob, and read the SWR, it couldn’t be simpler.  But the frequency scale is kind of crude, so you’re not  going to get anything really dead-on.  There is an output for a frequency counter which might help some, but the frequency adjustment knob scale can be pretty coarse at various band edges.  The chief thing is that this device will get you into the ballpark on antenna tuning without breaking the bank.  One more joker in the deck: it doesn’t come with a power supply, just a 12V input jack.  (The web site says it can use a 9V battery or 110VAC, what the heck?)  Luckily an AC adapter off of an old cell phone worked.

Here’s a link to the manufacturer’s site: http://www.mfjenterprises.com/Product.php?productid=MFJ-207

[armymars]

  I have one of the MFJ antenna analyzer that cost me $80 dollars. I take a post a note and make a new scale for the band I need using my ham receiver. This is good enough for checking out the antenna at the antenna for the band in question. 

[Carl]

I have ,and often use,a MFJ 259 analyzer...but unless you plan a lot of time designing and experimenting with antennas...I mostly cut wire to length and let the auto antenna tuner do the job of protecting the radio. You need a tuner ,whether you use an analyzer or not.

[armymars]

  If an analyzer can tell you how much and what kind of reactance is at the antenna, you can compensate for it. Cut the length per formula then find the resonate freq. with the analyzer. Now multiply the new freq. with the length and you have the new number to calculate the right length. Not for just this antenna, but others in the same location. Remember to subtract 5% for inverted "V".   

[Greekman]

  If an analyzer can tell you how much and what kind of reactance is at the antenna, you can compensate for it. Cut the length per formula then find the resonate freq. with the analyzer. Now multiply the new freq. with the length and you have the new number to calculate the right length. Not for just this antenna, but others in the same location. Remember to subtract 5% for inverted "V".

qot a question...is that -5% peculiar to the inverted V dipole?

[Carl]

qot a question...is that -5% peculiar to the inverted V dipole?

It is commonly accepted,but alway cut the wire a bit long,because it is really hard to add wire,
and just fold the wire back on itself after it is attache to the end insulators....folded back like
this the extra wire is not effecting the antenna tune.

[armymars]

  The antenna looks longer because of the interaction of the ends to the ground. This adds some capacitive reactants to the antenna.

[Greekman]

hmmm thansk for that....

it may be the qause why my wire 1/4 grounpalne seemed also longer when I was tuning/cutting it 3 feet of the ground only...

[Smurf Hunter]

I have used my internal tuner and swr tuner to locate the frequency my wire has the lowest SWR on.

Is that same frequency ever NOT also the most resonant?

e.g. a dipole shows lowest SWR at 14.3mhz

I know my radio will get the most from the TX watts on 14.3.  What other data could help?

[Carl]

I have used my internal tuner and swr tuner to locate the frequency my wire has the lowest SWR on.

Is that same frequency ever NOT also the most resonant?

e.g. a dipole shows lowest SWR at 14.3mhz

I know my radio will get the most from the TX watts on 14.3.  What other data could help?

Resonance is loosely associated with SWR though not as one might think.
How resonant is a DUMMY LOAD?   Yet it has ,or should have ,low SWR.
You don't TUNE an antenna by cutting or adding length...you tune the SYSTEM
to make the radio happy...nothing more.

Please read here ...again.

http://www.qsl.net/arrlsb/Digest/Pages/Antennas/antennas03.html

A quote from the article:

The Basic Half Wave Antenna
By “antenna” I mean the antenna itself and excluding the feedline attached to it. The first misconception hams have is that the closer the basic dipole antenna is to a half-wave length in the band of interest the closer it will approach perfection and the better it will radiate.

That belief is not supported by either theory or practice. Theoretically, the half-wave antenna does not radiate any better or worse than any other length. In other words, all lengths radiate equally well. You will not find any statement to the contrary in any reputable antenna text including the ARRL Antenna Book. Nor will you find any claim that the half-wave antenna maximizes radiation compared to other lengths.

So why does every antenna article ever published show the half-wave dipole antenna as the basic antenna? The answer is because it is the basic antenna. But this does not mean that it is the better antenna. In text books it is the basic antenna because it is the easiest length by which to describe mathematically and conceptually how antennas in general operate. For one thing the sinewave-like distribution of voltage and current along the wire can be drawn very nicely to exact fit on the half-wave wire. Can you imagine what it would be like if your basic antenna text book began by describing a 37/64 or an 9/17 wavelength antenna? It can be done, but the drawings and the mathematics would really by messy using such odd lengths. But a half-wavelength...well, how elemental can you get?

But why is almost every real-world antenna a half-wave-length long? Now we’ve come to the crux of the matter. The answer is because, in the practical world, the no-tune transmitter wants to see 50 ohms and the coax cable wants to see 50 ohms and the half-wave dipole impedance can be made close to 50 ohms. Everything comes out closely matched and everybody is reasonably happy. It is the half-wave-length antenna that makes all of this possible. This convenient accommodation to the demands of the transmitter, however, says nothing about the radiating efficiency of the antenna itself. Nor does it make this antenna length the best radiator. The only thing it says is that this length makes the SWR low at the transmitter. SWR describes a condition on the feedline. SWR says nothing about the radiating qualities of an antenna.

That a low SWR does not validate a good antenna is easily demonstrated. Consider a 50-ohm resistor terminating your feedline. The SWR is a perfect 1:1 but would you use that resistor as your antenna?

[Smurf Hunter]

Geez - I know the old joke that a dummy load will give 1:1.   
I fully understand that different antennas all with low SWR can perform differently.

Suppose I have a 10 meter dipole, and my target frequency is 28.4mhz.  And suppose I get lucky, and the length I cut gets a perfect 1:1 at 28.400. 
At 28.300 the SWR creeps up to 2:1, and same with 28.500 at 2:1.

Given all that, is it not also true that I'll have the best radiation pattern at 28.400mhz?

It seems worth knowing the "sweet spot" of any antenna system.  Seems that guys into QRP would spend a lot of time thinking about this.

[Carl]

Geez - I know the old joke that a dummy load will give 1:1.   
I fully understand that different antennas all with low SWR can perform differently.

Suppose I have a 10 meter dipole, and my target frequency is 28.4mhz.  And suppose I get lucky, and the length I cut gets a perfect 1:1 at 28.400. 
At 28.300 the SWR creeps up to 2:1, and same with 28.500 at 2:1.

Given all that, is it not also true that I'll have the best radiation pattern at 28.400mhz?

It seems worth knowing the "sweet spot" of any antenna system.  Seems that guys into QRP would spend a lot of time thinking about this.

READ CAREFULLY,I wrote it carefully.

But remember that a half wave dipole is NOT 50 ohms...(more like 73 ohms or 1.5 SWR)An inverted "V" is closer to 50 ohms
The 1 to 1 SWR actually can't be done with a resonant antenna....

So why is 50 ohms so important?,  because the radio is designed for 50 ohms and feeds best into 50 ohms...
Why 50 ohms?    Because of tons of low cost 50 ohm  feedline left over after the war.
Before that 300 ohm and 450 ohm ladder line were'the best' (for many purposes ,they still are best.

When you cut an antenna from the book or the equation...it is resonant.
When you cut or add wire to get lowest SWR...you are adding in loss to get a lower meter reading
(and better match to the radio) But NOT making the antenna any better .
A length of COAX will have LOSS and loss works both ways...
So crappy ,high loss coax will improve SWR readings(but does that make the antenna better?) NO.

You are NOT TUNING the antenna ,but adjusting the SYSTEM...
with all it's mismatched impedances and losses and interaction with ground and other conductors of RF.

PERSONAL NOTE SMURF:
When you tuned your antenna for better SWR ,it could better be described as adjusting the loss and resonance
of your HOUSE'S various conductors and their  interaction with your antenna system...your measurements will not
work the same for anyone.

The antenna will work better at the ORIGINAL LENGTH,the radio works better at 50 ohms,
This is why a Antenna Tuner (better named Matcher as it does not tune the antenna,it matches impedance )
is so important as a safety device.

Keeping the radio happy ,by matching impedance, is allowing the radio to feed maximum efficient power to the antenna and not cut back...
But all power does go out the antenna ...less heat in feed line and tuner(matcher ) components.
Antennas are indeed a confusing subject and that is why I cut wire to length and let my automatic matcher (LDG) keep the radio happy as I would rather spend time operating than crawling,climbing,cursing over a bit of wire.


Read here:

http://www.comportco.com/~w5alt/antennas/notes/ant-notes.php?pg=15

and here :

http://www.antenna-theory.com/antennas/halfwave.php

or trust me and spend your rare spare time operating.

I think I got all the above correct now.

[Greekman]

When you cut an antenna from the book or the equation...it is resonant.
When you cut or add wire to get lowest SWR...you are adding in loss to get a lower meter reading
(and better match to the radio) But NOT making the antenna any better .

So how can a guy armed with just a SWR meter achieve a happy medium?
Say cut to length and accept a bit higher SWR value instead of trimming for 1.1? Like 1.4?

and a related question. Should one that cuts the antenna at theoretical length factor in the wire velocity factor in addition?

[Smurf Hunter]

So how can a guy armed with just a SWR meter achieve a happy medium?
Say cut to length and accept a bit higher SWR value instead of trimming for 1.1? Like 1.4?

and a related question. Should one that cuts the antenna at theoretical length factor in the wire velocity factor in addition?

Most important question - are you running an antenna tuner?

If not, then you need to cut the wire to get pretty close to 50ohms else risk damage to the radio.

Otherwise cut to the frequency (468/FREQ=total length in feet) and let the tuner balance the impedance.

[Carl]

So how can a guy armed with just a SWR meter achieve a happy medium?
Say cut to length and accept a bit higher SWR value instead of trimming for 1.1? Like 1.4?

and a related question. Should one that cuts the antenna at theoretical length factor in the wire velocity factor in addition?

I will still adjust antenna length a bit,If the antenna is cut for a specific band ,
I try to center the 'sweet spot' on my most often used frequencies but I don't over worry it
as the effect on the effective output is minimal.

But my main point is antennas ARE NOT TUNED,they are de-tuned to make an SWR meter look good and match the radio ...The antenna does not work markedly better ....the radio does. I also stress the effect of lossy feedline causing FALSE SWR READINGS.

Here is a thought .A dipole is 73 OHM impedance and so at it's best will give a 1.5 to 1 SWR...
I can adjust (so can you) a dipole to read 1 to 1 ....does that make it better (NO,it does not).
So why will a properly cut dipole read LESS than it's TRUE 1.5 to 1?
Because of loss in the feedline.
Really bad coax will have GREAT SWR as it loses BOTH WAYS and less power gets read on the reflected meter.
The SWR meter can indicate antenna trouble...but also reads good with other ,output crippling .problems..

Don't judge an antenna by SWR as SWR is only one factor that makes a good antenna system.

When an antenna is in question ,I always measure output power ,to a dummy load ,at the radio and
at the end of the coax ,There should be very little ,if any,measured difference.(check against DB loss per footage of coax)
 Then,when satisfied the feedline is good ...add the antenna into the system to look at SWR, if that is not good...I tape measure the antenna and look for nearby metal,contact with trees , and length equal on both halves for antenna balance
and also look closely at the feedpoint or balun.

I do all this before adjusting wire length because they all have to be know as good
for SWR to mean anything more than 'SOMETHING IS WRONG'

Wire velocity has minimal effect on antenna elements for dipoles etc,
I always cut a little long (5%) plus wire length to tie on insulators.
I have never worried about it unless for coax used as antenna elements.
I admit,there are some things I just accept and move on,without trying to do a bunch of math equations.


I read ahead Smurf...good ,simple answer.You posted while I was still thinking. I shoulda' done that.