LF: Re: VLF G3XIZ - 12/13th March - Station Set UP - Advice Welcomed

["mal hamilton" <g3kevmal@talktalk.net>]

Chris

I set my system up as follows

 

Resonate my inv L system for the required frequency VLF or LF as required. I wind a loading coil as near as possible to the required frequency then connect in series with the inv L  then use a signal generator connected to a Scope to sweep the freq area for a resonance indicated by minimum amplitude deflection on scope, then add turns or remove turns to get resonance on my preferred frequency.

Min deflection also corresponds to min SWR but not necessarily 1:1 at this stage

Having done this LEAVE THE ANTENNA ALONE.

 

Next, match the transmitter to the antenna  with a ferrite wound transformer, the P and S windings depend on PA Z and antenna Z

A good starting point on a 3c85 type core is P - 9 turns and S 10 - 30 turns for a 50 ohm match.

My systems are all designed for 50 ohms which makes it easy for testing,

 

If you do not want to use 50 ohms at TX and Ant end then you will have to design a transformer that matches say 8 ohms to hundreds of ohms at the antenna end on VLF also some sort of matching atu at the base of the antenna to get the SWR down to 1;1  

 

Once the antenna is resonated leave it,  use an ATU adjustment to get the SWR down to 1:1

If you are having problems then its the matching transformer between TX and Antenna that needs attention.

I hope you get the gist of this description

mal/g3kev

 

----- Original Message -----

From: Chris Osborn

To: LF Group

Sent: Friday, March 11, 2011 1:05 PM

Subject: LF: VLF G3XIZ - 12/13th March - Station Set UP - Advice Welcomed

LF,

 

I shall be transmitting on VLF again over the weekend.

 

The main purpose will be to test (and adjust) my new loading coil therefor the transmitted frequency may vary.

I shall keep the reflector informed as to frequency and times that transmissions occur should anyone wish to take a look for my signals.

 

PRESENT STATION SET UP

 

TX

 

I have the option to use either an internal OCXO or an external off-air standard frequency, both of 10.000 . . MHz which is then passed through a programmable divider to give even divisions e.g.

10.000/1110 = 9.090 909 kHz    10.000/1112 = 8.992 806 kHz etc. . .

 

The PA is capable of giving several hundreds of watts but it has a low output impedance and the aerial system has a high impedance (several kilohms).

To achieve reasonable matching I use a large laminated-core mains transformer with a step up ratio of 1:24

 

LOADING COIL(S)

 

To tune out the aerial's capacitive reactance I use a series loading coil (inductor).

The original coil was single-wound, about 600 mm diameter and has an inductance of ~ 1.25 H

With the above TX, loading coil and my 40m inverted 'L' I can generate an aerial current of  0.15 - 0.2A

 

The original coil's losses seemed high so to improve matters I have wound another unit, this time using heavier gauge wire and of double-layer construction.

Initial results have been disappointing as I generate less aerial current than with the old coil.

 

The inductance necessary to achieve resonance has also fallen: from ~ 1.25H to ~ 0.75 H implying that the new coil has considerably more self-capacitance.

There are a few hundred turns on it which are not used and may be causing the problem so I intend slowly to remove these, checking the aerial current at intervals.

 

AERIALS

 

I generally use my 40m inverted 'L' aerial, height 10m but have the facility (wind conditions being suitable) to launch balloon or kite-assisted wires.

My property borders a public playing field so I need a wind direction SE - S  or SW for kite operations and near still air conditions for balloons.

 

ADVICE

 

I should be much obliged for information which will enable me to improve the station's ERP and detectability.

Can someone point me in the right direction to improve the matching system, maybe using ferrite cores which are readily available ?

Link coupling to the loading coil may be an option but my early attempts at this merely shorted out the loading coil and damaged the PA, so I have avoided it.

 

73

Chris G3XIZ