Finding resonance of an Antenna

[Gito]

Hy

I used to tune an Antenna with MFJ 259  antenna Analyzer,but in High RF environment,This Antenna analyzer cannot be used.

So I'm thinking to find the resonance of an Antenna with a GDO meter(grid dip meter,the Question is ,Can I use it in High RF environment.

Gito

[WQ9E]

Gito,

I think it was W8JI that posted some info on his website about using the MFJ analyzer near strong BCB RF fields.  You might google that if MW/LW RF is the issue.

The grid dip meter should work IF the RF field isn't both strong and on the same frequency as the antenna.  I have used my Measurements Model 59 GDO when setting the starting length for cubical quad loops prior to final tuning.

[Gito]

Thanks

Since the basic MFJ  analyzer is a vfo , with a resistance bridge meter for SWR,that used a rectified RF voltage with  a germanium diode ,
So if any high RF(all frequency) and high enough, is present around the Antenna,it was directly rectified and disturb the SWR reading..

Yes You are Right if the frequency is the same,maybe we can not use A GDO
meter.

The problem ,near the Antenna is a cellular Phone tower operating and I believed ,it has  enough power to disturb,the reading of my MFJ 259 antenna analyzer,

Gito

[N8ETQ]

   I have had luck (In some case's) putting
a known good Low Pass Filter in line with the
thing. HF transmitting type.

GL

/Dan

[Gito]

Hy Dan

That is what I;m thinking too.but in the Other hand low pass filter changes with the impedance of the load.since low pass filter values depends on the impedance of the Antenna( which we are trying to tune to 50 Ohm).

Why i I am trying to used a GDO ,because it easy to build ,and now days we can buy a "cheap" frequency counter,
using this two in combination.we can find "Exactly" the resonance  frequency of the Antenna.

Gito

[flintstone mop]

Another method might be to connect the TX, very low power, and with a field strength meter adjust the tuner for peak reading. OR look at the SWR meter of the transceiver as you spin the dial. What freq are you dealing with? Ham radio? Or broadcast?
You did not mention what is nearby to upset the MFJ. What freq and what power? Cell phones are 700mhz to microwave frequencies. That would not couple any RF into your MFJ.

[W3RSW]

Similarly, for years I've just fired up a rig like a tr7 that can transmit about anywhere on the bands, heh, heh, at the 5 watts (heh, heh, again) and just plot the SWR rising or falling as you change freq.  Then, adjust the antenna length proportionately. Very easy math and cutting or folding back to get element length(s) desired.

[Gito]

hy

I don't think the offending RF Frequency must be MW/LW strong signal that disturb the SWR reading.
Any RF  frequency which is High Enough around the antenna can disturb the SWR reading .look at the MFJ 259 schematic. The SWR meter is build of R16,R17,R18 ,and the Antenna connector .

It's a resistance bridge ,where R 16,R17,R18 = 49 Ohm ,and a germanium diod ( D3=1N314) is placed to rectify the balanced between this bridge.

Look at this dioda .it is directly connected between R 16 and the Antenna terminal.
So any high  Rf voltage pickup by the antenna goes directly too this dioda and rectified as a voltage ,this affect the balance of this SWR  bridge.
And disturb the SWR reading


Gito

[flintstone mop]

OK Gito
What high power frequencies are near your antenna?

Fred

[Gito]

Hy Fred

That is the Question,I only know that the SWR meter  reads very high SWR and also The Resistance meter reads High resistance (full scale).

What I know is a BTL tower (cellular tower) very  near (about  70 meter far)  My Antenna ,

Can also because of overloaded of the SWR meter
Since  this SWR meter can behaved like a Crystal radio?
Or a field strength meter ?

and in the other hand I succeeded in finding the resonance of many  FM broadcast radio in other places ,using MFJ 259 and also MW vertical Antenna (by modifying the coil in the 1.8 - 4 Mhz band in the MFJ 259}

 to tell You the truth I am trying to tune a 40 meter tall vertical for Medium wave near/close  this Cellular tower .


Gito


 

[W7TFO]

Find/rent/borrow a Delta OIB-1 or 3 operating impedance bridge and the companion RG-3 or 4.

They will do the job in very high RF fields.

http://www.deltaelectronics.com/data/oib1&3.htm
http://www.deltaelectronics.com/data/rg4data.htm

Too bad you are in Europe or you could use mine.

73DG

[Gito]

Hy W7TFO

Thank you for the links You Gave Me,i 'm reading this links and try to understand how this QTB works.

I've been thinking  to place a high Q  variable parallel resonant circuit after the output of this MFJ 259 with a dial calibrated where the parallel frequency is resonance.

So for example if we used the MFJ 259 at 1.4 Mhz ,than we turn the Parallel resonance  dial to 1.4 Mhz

not like a band pass filter that depends on the impedance of the load and the "transmitter"to work Correctly ,

A parallel resonance circuit has a high impedance at the resonance frequency ,thus not loading the MFJ 259,but it by pass all the frequency  that is not the resonant frequency.

Can it work?

Gito

[John K5PRO]

The Delta Electronics OIB-2 operating impedance bridge is the right one for HF use. They are rare but were made to measure inline vector impedance with up to 1 kW of operating power. With higher power you don't need their RG3 or 4  with the synchronous generator/receiver. Just null the meter with the knobs and read your Z.