Survivalism & Self Sufficiency Topics > Amateur Radio Gear Reviews

MFJ-207 Antenna Analyzer

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Smurf Hunter:
Geez - I know the old joke that a dummy load will give 1:1.    :P
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:

--- Quote from: Smurf Hunter on March 24, 2015, 10:12:01 AM ---Geez - I know the old joke that a dummy load will give 1:1.    :P
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.

--- End quote ---

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:

--- Quote from: Carl on March 24, 2015, 11:21:53 AM ---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 .

--- End quote ---

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:

--- Quote from: GreekMan on March 24, 2015, 01:07:10 PM ---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?

--- End quote ---

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:

--- Quote from: GreekMan on March 24, 2015, 01:07:10 PM ---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?

--- End quote ---

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.

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