TV modulator mod?

[luRaichu]

I'm attempting to build a powerful, analog TV transmitter that fits inside a briefcase. At least 5 or 10W, hopefully more?
Most builds I've seen use a consumer grade RF modulator, which limits the TX range to channel 3/4. For other channels you need an agile modulator which is huge and probably overkill for constant TX on one channel. Also, I want MTS, but my Olson agile modulator doesn't support it.

So, how hard would it be to modify one of those mini-modulators for channel 2 operation? I'd imagine it would involve swapping a crystal at least.

And, changing the RF output impedance from 75 ohms to 50 ohms?

[jhalphen]

Hi to all,
Hi LuRaichu,

a consumer grade VHF TV modulator is a "dirty" device :

For one, it uses full dual-sideband modulation vs a Broadcast quality unit like the Blonder-Tongue generates a Vestigial sideband TV signal which is compliant with broadcast standards.

What this means in practice, is that a consumer grade unit will "pollute" the channel below & above the selected channel.

Furthermore, consumer units have no or minimal RF filtering of the output. Fine for connecting by cable or very low power local transmission but amplify THAT to 5 or 10W of RF power you may seriously pollute the RF spectrum at frequencies very far from your intended channel.
Example : Ch 2 is in the 45-50MHz band, have some spurious output at harmonic 2 in the 110-136MHz aircraft band = guaranteed pronto visit by FCC & police (interference to aircraft comms).

Anyone interested in significant RF power output (beyond 1W) should have a spectrum analyzer to ensure "cleanliness" of his signal.
Units like the TinySA Ultra are a revolution; for 120$ you achieve more/less what a HP analyzer did for 20-50k$ 30 years ago.
Note : buy it from a SERIOUS source, most Ali-Express/E-Bay units are fake copies.

https://tinysa.org/wiki/pmwiki.php?n=Main.HomePage
There is a "where to buy" page.

in the 90s, i used professional modulators & Avantek or Minicircuits RF power amps with 1W power.

Avantek is defunct but MiniCircuits is still alive & well :

https://www.minicircuits.com/product...mplifiers.html

I don't know what's your project (pirate TV for an entire town?) but a 100mW BT modulator of the AM-60 Series comfortably covers a house/property. They are still cheap & cheerful secondhand on E-Bay.
Many of us here use them as low power personal TV transmitters.

Blonder-Tongue AM-60 Series search on E-Bay/PicClick:
https://picclick.com/?q=Blonder+Tongue+AM60%2A

Best Regards
jhalphen
Paris/France

Avantek UTC SEries wideband RF amplifier Series spec sheet.

[luRaichu]

There goes the briefcase transmitter...

Originally Posted by jhalphen

I don't know what's your project (pirate TV for an entire town?)

Yes, that's what I think it is.
I started with a $7 chinesum amp from Amazon or Evil-Bay, fed by my agile modulator. Rated 2W output @ 12v, about 1.5W @ "safe" voltage of 10v. Lovely harmonics. Filtering is a must!

The big problem is that there's impedance mismatch between the 75 ohm modulator and the 50 ohm amp input. What transformer would you suggest?
I know RF-Links has 75 ohm TV amps though. I'll have to see if they're within my budget.

[Electronic M]

Originally Posted by luRaichu

There goes the briefcase transmitter...

Someone either hasn't seen the dimensions of an AM-60 or has already bought an extra small briefcase....My AM-60 is 14.5"D x 1.75"T x 19"W....It can be 17.5"W if you cut off the unnecessary rack mounting ears. There's definitely briefcases out there that can fit an AM60 and a thin battery, power inverter and media player underneath it.

A BAVMz can transmit about 3/4 block (AM-60 is one block) with only a good antenna (dipole cut to frequency), and a BAVMz is about 9" less deep than a AM-60. Most briefcases should be able to fit 4 BAVMz family modulators...Now the BAVMz is fixed channel (unlike the agile AM60s), but you can buy ones built for any VHF channel and all cable channels (some of which overlap UHF TV channels).

Another thing to consider is what channels you want to use and how big your antenna must be to be decent as a TV transmit antenna. The best solution for simplicity and Omnidirectional operation is a dipole cut to 1/2 the channel wavelength...For channel 2 that's a nearly 10 foot long wire with a coax connection in the middle! It gets smaller the higher the channel though... Channel 13 is about 26" (you could build a couple of telescoping rods that retract into the briefcase), and a standard 1970s era UHF bowtie antenna (so far my favorite for UHF receiving and transmitting) is about 13".....
Generally using an unnecessarily high power level to get a certain amount of coverage from a bad antenna doesn't yield as good of results on the receiving end (or for the reliability/life span of the transmitter) as using a good antenna with sufficient power. If the transmitter antenna isn't tuned for the channel you can have almost all the RF power not radiated into space and instead reflected back into the output stage where components will see ~2x their designed power level (the power their sending now plus the full power they sent a fraction of a wavelength ago) and rapidly burn up....In some cases TX life expectancy under those conditions can be measured in seconds. Also if almost all the power is being reflected a 2W TX can have worse signal coverage than a much smaller 100mW transmitter.

I think your project is doable and interesting, but it requires sound engineering effort....If you brute force design it you'll be building a glass jackhammer that works worse than a steel carpentry hammer.

[jhalphen]

Hi to all,
Hi Electronic M,

Thanks! for the additional info about Blonder-Tongue models.

Broadcast antenna:
Here are Phil Nelson's info to build a VHF Ch2 to Ch13 dipole :
https://www.antiqueradio.org/art/VHF...oleLengths.jpg

Main article about home TV broadcasting :
https://www.antiqueradio.org/HomeTVTransmitter.htm

Thanks! Phil.

BT AM60 Series: i have the user manuals but no schematics.
BTW, anyone have access to them?

Remember reading the AM60 internal power supply uses a + and - 15VDC rail*.
In Lu Raichu's project, it would be easier to provide +/_ 15VDC from batteries rather than batteries + 120VAC inverter + reconvert to DC.

*filter capacitors needed replacement as BT units were on 24/7/365 in cable TV headends.

Best Regards
jhalphen
Paris/France

[Electronic M]

Jhalphen I had a similar thought on eliminating the AC supply (IIRC it occupies the right ~2" of the cabinet and could be removed to save space but I feel like I saw a 30v line in my AM60 when I was in it last year but I can't remember well enough to be sure.... There's many regulated power rails and at least 2 bridge rectifiers on an AM series (one of mine blew in a storm) so replacing it with batteries isn't marvelously straightforward.
I feel like the BAVMz series has only a 12V rail, but I haven't opened any of mine in 7 years so that's a less reliable memory...I need to do it soon though the audio and video F connectors have been dirty and intermittent for years on mine (I mistakenly blamed the cables for a long time) and I'm going to eventually replace those with RCA jacks so I can use the F to RCA adapters currently on it elsewhere.

[luRaichu]

Where do I find more information on BAVMz?

[jhalphen]

Hi to all,

@Electronic M & Lu-Raichu,

Here is the Blonder-Tongue analog modulator product page :
https://www.blondertongue.com/produc...og-modulators/

I do not see the BAVmz range, probably a legacy product.

Are these small plug-into-a-rack models, 6 or 8 to a row or a 1U 19" unit like the AM-60-xx Series ?

Do you (or anyone) have perhaps Blonder-Tongue AM60-xx schematics? paper or pdf?
i have 2, would like to have for future maintenance.

Best Regards
jhalphen
Paris/France

[Electronic M]

The BAVMz are 1U 19" units, but they're very shallow.

[jhalphen]

Hi to all,

@Electronic M + @Lu-Raichu,

BAVM is not easy to find, see here Blonder-Tongue 2013 catalog + page capture.
Page # is 80. Max RF output level quoted as +55dB/mV

https://www.yumpu.com/en/document/vi...boratories-inc

You can download, but need to sign in. i don't do that.

Unit is single channel factory programmed, can't change, so entirely dependent on what the cat dragged in from the defunct CATV headend.
Considering the low prices of obsolete AM60-xx Series, i would stick with an agile (programmable) modulator.

New ones here, VHF, Ch 2-36:
https://www.fullcompass.com/prod/094...r-channel-2-36

UHF channels 37-94 :
https://www.fullcompass.com/prod/156...-channel-37-94

see pdf spec sheet attached.

Best Regards
jhalphen
Paris/France

[etype2]

Excellent! This thread should be a reference guide for anyone interested in the subject.

[luRaichu]

Does the composite video source matter as much as the modulator? I've heard stories about old school pirates running the station off a consumer VCR.

[jhalphen]

Hi to all,

Thanks! E-type2

@Lu Raichu : any analogue source can feed the modulator : NTSC composite output of a digital ATSC box, VCR, DVD player, video camera, test pattern generator, computer,...

"Old school pirates" probably used the VCR as an edited program source.
The VCR's modulator could feed a distribution system (such as a block of condos) and be received as a channel on cable.
However, the VCR modulator connected to an antenna will "Broadcast" to a distance of 3ft-6ft max (and with a snowy picture).

The Blonder-Tongue(s) discussed above have a far more powerful RF output especially the AM-60 series. 60 meaning 60dB/millivolt.

VCR modulators or the classic Radio Shack which outputs on VHF Ch3 & Ch4 radiates 60 to 90dB/microvolt, enough for a coaxial feed direct to your TV, not for OTA Broadcasting.

See attached file giving RF level measurements according to the dB (deciBel) reference level chosen : microvolts, millivolts, power in milliWatts. Impedance is 75 Ohms in TV gear. In radio, it's 50 Ohms. This is important when measuring actual RF power.

Best Regards
jhalphen
Paris/France

[Electronic M]

For the most part it doesn't matter what composite source you use. There are two exceptions IMO.
Most VHS VCRs lack an internal TBC so when playing back tape you can get flag waving bends at the top of the picture and other distortions... There's also macrovision (which is present on most commercially released tape and disc media) which violates legal NTSC composite video values and plays havoc with most vintage TVs and other equipment used to view it. (There are macrovision remover boxes out there that are about the size of a deck of cards.)
The other source I would consider avoiding is the cheap plastic cased Chinese HDMI to AV converters and the similar HDMI to RF converters on Amazon, eBay and the like. They often have no color and video performance issues.

Most VCRs would be pain to fit in a briefcase and have room for a decent external modulator and or power source.

What I would probably use if I wanted a media player in the modulator case is a Sony SMP-N200 network media player. They take 12V DC power, have composite output, can play photos audio and video files off of a large USB drive and also off of a network through WiFi. They're about 6" x 6" x 2" so you could probably squeeze it in somewhere easier than most other sources... Except maybe a raspberry pi which I think are smaller.

[luRaichu]

Originally Posted by luRaichu

I know RF-Links has 75 ohm TV amps though. I'll have to see if they're within my budget.

I just called RF-Links and they want $1000 for a 2 watt TV amp... I'm sure the 30 watt one costs more. And, I think they actually use 50 ohm input impedance.

Guess I'm stuck with my 2 watt chinesium amp for now. I can't find any better/higher wattage amp designs that will work on the VHF TV band.