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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

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Old 25th Apr 2021, 11:51 pm   #21
majoconz
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Default Re: Spectrum Comms. Active Antenna problem

You already said that you have connected all your radios to the loop and amplifier and every one was overloaded.
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The Spectrum antenna has been connected to every different radios here for testing. But they all were the same = overloading, and the MW signals appearing all over, as soon as power is on the antenna.
Therefore the the only common factor is the loop and the loop amplifier. IMHO the MW band signal is cross-modulating everything in the loop amplifier - therefore a high-pass filter with a cut-off of about 1700kHz should block all the MW signals - or at least reduce them to a tolerable amount - between the loop and the amplifier is your best bet. You may need more than one filter to accomplish the required rejection.

https://www.minicircuits.com/product...s-Filters.html

This one for example will only block by 20dB above 1200kHz.... ZFHP-1R2-S+. You may need the next one on the list .... ZFHP-2100-S+
Lumped LC High Pass Filter, 2500 - 6800 MHz Connector Type: SMA .....but you will lose the ham top band.
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Last edited by majoconz; 26th Apr 2021 at 12:00 am.
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Old 26th Apr 2021, 12:25 am   #22
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Default Re: Spectrum Comms. Active Antenna problem

Radio Wrangler said:
Quote:
The NRD525 was one of the better amateur receivers of its day in terms of large signal handling, but it was not as much better as legend has it.
I have an NRD515 fed from a multicoupler - I have never suffered from cross-mod or any other artifacts - maybe because....

A Reaction Instruments 409-2 VLF/HF multicoupler feeds my HF receivers. The input amplifier is an MRF136 power MOSFET - max power rating 55 watts. You will leave your fingerprint on it if you touch it while it is on!

My Redifon R499 receiver has a push-pull RF amplifier after a preselector single channel filter. This uses four 5 watt 2N3866's and is practically bomb-proof! Unfortunately I don't have the crystals for it!

So the recipe for an RF amplifier is adequate filtering before any gain and good linearity with lots of headspace!
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Old 26th Apr 2021, 2:02 am   #23
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Default Re: Spectrum Comms. Active Antenna problem

At HP we were involved with selective level measuring sets which had to winkle a single channel out of thousands, all thundering away at max level loaded with phone calls. We ran into problems trying to get crystal filters with adequate dynamic range to not be the limiting factor. 2N3866 and custom arrays of >5GHz bipolar power devices were used. The mixers were designed by Hugh Walker (Author of "Sources of intermodulation in diode ring mixers" Proc IRE)

Later, we made an NPR test set which used a superhet receiver with a next generation diode ring mixer (Very high level!) and the channel selectivity at IF. This worked with an FDM system where normal loading was simulated by white noise at max level across full system bandwidth. We could reliably get a slot depth of comfortably over 70dB, but for best NPR floor, we needed to have crystal preselector filters made. In the end, it looked like only Thomson-C.E.P.E. could make them linear enough.

After that sort of development, pushing off-air receivers to better large-signal handling and dynamic range wasn't exactly easy, but it was easier. The tale of the NPR measuring set is in the HP Journal for April 1982. The instrument was too late for the market. No more FDM was being deployed, PCM was taking over. What was learned got used, though when later we got involved in surveillance receivers for spectrum management agencies, and some other people.

Great fun!

David
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Old 26th Apr 2021, 2:47 am   #24
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Default Re: Spectrum Comms. Active Antenna problem

Hi David - I might have one of your toys here, although you may weep when I tell you how much I paid for it - a 3586A SLM in non working order - NZ$120, say GB£55! I'm not too sure whether it's an A,B or C - the NZ POTS must have had it made to order, although the POST says it's an "A" in the firmware. The dummy who installed the rack mounting plates used screws that were too long which bit into the display panel PCB and grounded a 5volt rail! As a result of that, the Molex connector from the big transformer to the power board had overheated and distorted. The bottomless junk box provided the parts and a couple of hours fixing the front panel switches and replacing the audio pot (how do you shear off the shaft?) and it's all good. The hi-stab oven is now my frequency reference!
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Last edited by majoconz; 26th Apr 2021 at 3:00 am.
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Old 26th Apr 2021, 9:21 am   #25
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Default Re: Spectrum Comms. Active Antenna problem

That was originally designed as a selective wavemeter, more of a general lab instrument (3586A) than a telecomms unit, in Loveland CO. We added all the FDM bandplans and changed a fair bit of hardware to suit the SLMS (as the 3746A) role and it worked well.

It was the only SLMS with a crystal filter. 50.015625MHz two stages of lattice section in the first IF. The crystal manufacturer Loveland had found that could make crystals which stayed linear to quite high levels still had difficulty making them. The second IF is at 15.625kHz and all the filters are pot core based. The 3746 has a further conversion and a 22Hz active filter for measuring pilot tones.

I did a variant of the 3746A with a frequency extender box to cover to 90MHz. Note that on telecomms boxes, suffix 'B' was not a later version. An A suffix meant to CCITT standards, a B suffix meant Bell.

No, I won't weep. I saw the bulldozers go in and flatten the place where so much was designed and made.

Anyway, the SLMSs are very serious kit in terms of large signal handling. With switched IF gain in large chunks, they don't make good HF receivers (I tried, of course!) The OCXO may well be the most valuable thing there. The synth phase noise is quite decent but it covers 50.015625 to 85-ish MHz. Not quite an octave, so that spoils any idea of dividing down as a low noise sig gen.

But, in the front-end circuitry, there's a fair bit to be learned about high dynamic range design, and that can be applied to substantially less esoteric problems.

The NPR box was 3724A + 3725A + 3726A It was split into units which could be carried.

I had access to all the gear needed to thoroughly measure RX performance, and I managed to borrow and test all sorts of receivers. This meant I could test them in a uniform way. Manufacturer's data sheets always spec'd things slightly differently in ways which couldn't be directly compared. Different reviewers had their own pet conditions. So with that background, I know the JRC receivers and how they compared to the state of the art at the time. The Japanese firms tended to put slightly better receivers in their transceivers than they put in pure receivers. They saw the shortwave listener market as less demanding than the transceiver market. JRC seemed to do the receiver first, and then develop a transceiver from it.

Anyway, it's the active antenna that's the limiting factor for the OP.

David
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Old 26th Apr 2021, 12:01 pm   #26
Ian - G4JQT
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Default Re: Spectrum Comms. Active Antenna problem

As others have said, it seems you are either in an area with some remarkably strong AM signals that are overloading the antenna amplifier, or there is a fault in the amplifier, or possibly poor connections to the coax.

One thing to check is that it is getting the correct voltage.
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Old 27th Apr 2021, 9:57 am   #27
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Default Re: Spectrum Comms. Active Antenna problem

wow, thank you for the great info and advice on this issue with the antenna.

I had the Covid19 vaccination on Saturday, and it made me feel extremely tired. I didn't do anything on radio. Will slowly pick it up and go through all the messages, and try to resolve the problem. It looks like it will be a slow and time taking process to troubleshoot this antenna.

Thank you & 73s
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