48 watt VHF!

[MCW]

Anybody got any ideas on this one?
An ancient and venerable Navico RT6100 when transmitting on high power the set switches rapidly between Tx and Rx. When connected to a SWR meter it gives an output of 48 watts! and the reflected is about 5 watts. (The set works fine on low power and gives a shade over 1 watt). I dont think the airel glows, I've not looked at night but there are never any gulls around the masthead though!
I did try the man at simrad, but when I spoke about the SWR meter he went rather vague and said he had not heard of one - and he WAS the technical guy!
When I ask anyone in the trade they go silent, then there is a sharp intake of breath followed by 'that is an old set, not made any more, don't know anyone who repairs them now', so I have a feeling its time to consign it to the deep but all suggestions (legal) are welcome.

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[tr7v8]

Post a message to Oldgit normally on Mobo forum, he knows a bit about radio or so I'm told /forums/images/icons/smile.gif

<hr width=100% size=1>Jim

Draco 2500

 

[Strathglass]

An SWR meter will not measure power accurately unless it has been specifically designed and manufactured to work within the frequency range it is being used at. Even then it will only be accurate into a dummy load.

Try the SWR meter on another VHF and I think you will find that it will also give a high reading.
My guess is that you are using an old CB SWR meter which is intended for 27Mhz not VHF. This will give an indication of relative power between different VHF sets. It will probably be reasonably accurate when reading VSWR as this is the ratio between two measurements and not an absolute reading.

The SWR of your aerial system sounds very good..

Iain

<P ID="edit"><FONT SIZE=-1>Edited by iainsimpson on 15/07/2003 07:45 (server time).</FONT></P>

 

[john_morris_uk]

Iansimpson is correct. In fact measuring RF power with extreme accuracy is difficult enough with professional equipment! Most VSWR meters are frequency dependant and most 'CB' meters were notoriously innaccurate at measuring power anyway. (The frequencydependant bit meaning the higher the frequency the greater the reading). When I worked professionally in RF I used a Bird Thruline power bridge into an accurate dummy load, and even then it was only within a few percent, which is what your reading is anyway! On a more positive note, check the supply to the set. Some of the sets cycle on and off when the supply is low. Try it into another antenna if you like, just to satisfy yourself that you have eliminated the problem. but the VSWR sounds quite reasonable.

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[steve_cronin]

Dedicated Marine SWR Meter

.. weekend loan available (UK only) for a contribution to RNLI & suitable security safeguards to ensure return after use.

Steve Cronin

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[MCW]

Thanks for the ideas.
The set has been tried on both the ships main antenna and a Vtronix emergency one with the same results.
The SWR meter shows 25w when connected onto a friends icom set.
On the subject of frequency, I tried on ch06, would another one be better?

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[brianrunyard]

Check the output power with a 50 ohm dummy load not the antenna. What sort of quality is the meter you are using?

<hr width=100% size=1>Brian
http://homepage.ntlworld.com/brunyard

 

[stevebirch2002]

If you get the radio to me I will test it and let you know. Work in an RF workshop with hundreds of thousands of pounds worth of test gear from HF to 40GHZ..

I am located in Malvern but travel to The Solent very often.



<hr width=100% size=1>Albin Vega "Southern Comfort" V1703

 

[brianrunyard]

Steve
Hope you're going book it to the right Cost Code.
Brian

<hr width=100% size=1>Brian
http://homepage.ntlworld.com/brunyard

 

[IanBBA]

Suggest very strongly that you do not use this radio.

From what you have written, I believe the final power amplifier in your radio is becoming unstable and producing high power spurious emissions. It is very important that you do not leave this radio connected to an antenna as these emissions are likely to interfere with other radio users. It is even possible, dependant upon the radio design and I am not familiar enough with the circuitary of this one, that some spurii may be produced in receive mode. This amplifier is also unlikely to last long producing 48W!

Spurious emisions have many causes are uncommon in marine band equipment due to the very high standard of engineering required by the RA type approval process. Faults do occur however. Often installation factors play a part.

Spurious emisions are rogue signals transmitted along with the wanted signal. A common spurii produced in all amplifiers in some quanity or other is the harmonic which appears at 2x, 3x, 4x ... etc times the wanted signal and these are always reduced to extremely small levels by a filtering circuit.

The spurious signal I beleive your radio is producing is caused by the output amplifier transister not only acting as an amplifier but by some instability, also acting as an oscillator and producing an addtion signal or group of signals at high power levels. There could be a number of causes for this:-

1) A fault in the radio.

2) That antenna is not preforming poorly if the 48out/5in is a true reading (VSWR=2:1). The VSWR meter is a crude instrument as it is not frequency selective measuring the sum of all signals in either direction and for a number of reasons associated with that and your spurii, these reading you have may be false. Poor VSWR may be stimulating the power amplifier self oscilations.

3) Poor DC wiring or high levels of radio frequency signals being 'received' by the DC cabling. DC wiring should be as short as possible to the battery and voltage drop should be kept to a minumum.

My advise is:-

1) check the power wiring by connecting a digital volt meter to the 12V supply at the radio (as close as possible) and observe the voltage change between receive and transmit. The voltages with no charging system active should be something like 12 and 11.5V.

2) Check the antenna connector at the radio end. Both the inner and screen connection must be in good condition.

3) Get the radio checked out by a workshop.

4) Check the antenna VSWR but you need some 'isolation' at the radio ouput. Borrow something called a 6dB or 10dB power attenuator (rated at 25W) from your local friendly radio expert and place it in the antenna lead between the radio and VSWR meter. This will ensure that the radio always sees what it thinks is a good antenna and if your spurii are VSWR related, will stop them. Your VSWR meter will now show a lower transmitter power (2.5 or 4W) but the VSWR measurement will be valid. If your antenna is at the mast head, the VSWR should be no worse than 1.3:1. It the antenna is at or near deck level (ie short coaxial cable) it should be no worse than 1.5:1


Good luck, Ian.



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