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Author Topic: Matching 75 to 50 ohm coax for a preamp  (Read 2633 times)
VY1MR
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Posts: 8




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« on: November 22, 2012, 09:16:51 PM »

Here's my situation;

The CBC stopped broadcasting on MF here and I live too far away
to receive their VHF transmitter.  I built a little dipole and it helps a bit
but not enough.
I'm going to build a VHF preamp based on the design in the link below.
It uses a MAR-8 MMIC amplifier, which has Z0 of 50ohm.
Because this is for a consumer grade HIFI receiver, the input for a VHF antenna is 75ohm (unbalanced).

So my question is, do I need a 2:3 unun between the receiver and the preamp or will the loss
from the mismatch be small?.   
Thanks in advance


http://users.belgacom.net/hamradio/schemas/preamp_HF_VHF_UHF_SHF_wideband_MAR6.htm
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AC5UP
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« Reply #1 on: November 22, 2012, 10:12:12 PM »

I wouldn't worry about it.  With my luck the UnUn would be lossier than the mismatch.

The impedance of coax isn't that precise to begin with and in a receive application there is usually more than enough gain to compensate for what should be a very slight loss. I added a preamp for over-the-air digital TV and found the path of least resistance was a commercial cable TV drop amp.

15 dB gain with 1, 2 or 4 outputs, die-cast aluminum weatherproof case, DC power through the coax, not too expensive and rated for 50 MHz through 1 GHz bandwidth. A few years back a spec like that would have been Buck Rogers territory.............
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WB6BYU
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« Reply #2 on: November 22, 2012, 10:29:04 PM »

The MiniCircuits applications notes for the MAR-series amplifiers include a section
on their use in 75 ohm systems.  The gain is about the same as in a 50 ohm system,
while the return loss (SWR) is a bit worse, as would be expected.  Overall it probably
won't make much difference in your application.


The most critical parameter for your application may be noise figure rather than
gain.  The MAR-8 is rated at 3.3dB, while there are a number of modern circuits that
will be better than that.  The Hamtronics preamps are rated at 1dB or better noise
figure, but I don't know if they offer one specifically tuned for VHF FM BC.  (Their
120 MHz preamp could be tuned down with a bit of added capacitance.)

Other possible devices include the BF998, as shown here:
http://lea.hamradio.si/~s53ww/4xbf998/4xbf998.htm
a grounded-gate U310 (1.5dB), the MPS901, and many other options.  If needed, a
low noise preamp could be added ahead of the MAR-8.


Having worked for many years receiving weak TV signals over the air in a rural
location, the most important factor appears to be antenna height, next is
antenna gain, and then the preamp noise figure.  For best results all three
need to be improved.  Unfortunately, wide-band FM is not a very good weak
signal mode (though perhaps better than TV in that regard).  If you are already
getting a signal but it just isn't good enough, then adding a preamp may help
(depending on the noise figure of the receiver.)  Building a gain antenna of some
sort and getting it up as high as possible should also be considered - they may
take more work than building a preamp, but may also give better return for the
investment.
« Last Edit: November 22, 2012, 10:41:50 PM by WB6BYU » Logged
G8JNJ
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Posts: 473


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« Reply #3 on: November 23, 2012, 12:51:57 AM »

Hi,

I have tried using the MAR series of broadband devices for similar applications in the past.

In this case you don't need to worry about the impedance mismatch. It will make very little difference to the overall performance.

However I wouldn't recommend using broadband devices for this application, unless you put some form of bandpass filter ahead of them. As they don't really have a sufficiently high enough IP3 and will produce loads of in-band intermodulation products. Especially if you have it connected to a band II dipole in a typical urban environment.

By the time you add in the complexity of a bandpass filter. It would be simpler and more effective to build a much lower noise narrow band amplifier with a tuned matching circuit.

My suggestion would be to improve your antenna by adding a reflector and director to the dipole, to make it into a three element beam. Use satellite grade 75 ohm coax with a foil & braid screen. Fit a single clip on ferrite over the coax where it joins the dipole.

Then try a preamp with a gain of no more than say 10 to 15dB (to avoid overloading your RX) but with as low a noise figure as possible. (Ideally less than 1dB).

There are lots of websites dedicated to DXing FM broadcast stations. Which may be able to give you more hints & tips.

Hope this helps.

Regards,

Martin - G8JNJ

www.g8jnj.webs.com



 
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KCJ9091
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« Reply #4 on: November 23, 2012, 06:57:55 AM »

What polarization is your dipole?
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WB2WIK
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Posts: 20542




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« Reply #5 on: November 23, 2012, 08:11:17 AM »

In every case I can think of, I'd skip the preamp and use a better antenna.  Always.

If your receiver is actually any good, it may already have an RF preamplifer internally that's just as good as the external one you have planned, in which case the new preamp won't accomplish anything.  But providing "more signal" from the antenna (prior to any amplification) will always accomplish the goal.
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KE3WD
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Posts: 5694




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« Reply #6 on: November 23, 2012, 08:23:47 AM »

How about designing and building a Yagi for the frequency of interest? 

Nothing spectacular, likely a 3 element would do the task, since you already can hear the signal using a dipole. 

And since the station of interest is fixed position, once the Yagi is pointed and locked down, done deal. 

HINT -- Old Yagi TV antenna can be pressed into service here, and you may not even have to modify it to get the job done. 


73
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AC5UP
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« Reply #7 on: November 23, 2012, 08:50:20 AM »

In every case I can think of, I'd skip the preamp and use a better antenna.  Always.

...but shouldn't there be some consideration of the mode?

For instance: With analogue NTSC TV it wasn't unusual for a mast mounted amplifier to add a hint of noise to the picture. No one thought anything of it because they were typically used in a weak signal situation, but even on stronger signals you'd see vivid colors 'crawl' thanks to the preamp noise figure. And what about the audio? Clean & clear with or without the preamp as long as the FM detector had enough signal to reach limiting. All FM tuners have a spec for AM rejection and that's something of a measure of noise rejection as well.

Point being that AM & SSB are more affected by the noise figure of a preamp than FM & Digital. Since noise is amplitude modulated and FM offers "no static at all" as the lyric claims, NTSC video as a   w - i - d - e   AM mode would be far more sensitive to preamp noise than FM radio.

If I read the original question correctly it looked like VY1MR is interested in FM broadcast. Might not reach full quieting in stereo, but a preamp shouldn't add any significant noise to FM mono..........

BTW:  Ancient history, but when FM Stereo was approved by the FCC some broadcasters resisted the upgrade. Back then AM had the advantage of covering large chunks of real estate while FM was more local. To an advertiser like Campbell's Soup that mattered. FM Stereo cut the usable range in half (rule of thumb) and in some markets covering multiple cities stereo meant reducing the potential number of listeners, not better sound. Most people never try this, but a decent FM tuner with a passable antenna can do some nice work in mono with the muting turned off.
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WB6BYU
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« Reply #8 on: November 23, 2012, 11:04:05 AM »

Quote from: AC5UP
In every case I can think of, I'd skip the preamp and use a better antenna.  Always.


I don't think I was the one who made that comment.  It's too brief and concise. Grin



Quote

Point being that AM & SSB are more affected by the noise figure of a preamp than FM & Digital. Since noise is amplitude modulated and FM offers "no static at all" as the lyric claims, NTSC video as a   w - i - d - e   AM mode would be far more sensitive to preamp noise than FM radio.




I think you are confusing different things.  FM isn't prone to AM noise (such as
ignition noise) as long as the signal-to-noise ratio (SNR) is high enough for full limiting.  
But for weak signals they are often more difficult to demodulate than AM at low SNRs.  
That's why we use SSB or CW for DX work on VHF rather than FM.  FM signals are
quite noisy when they aren't strong enough for full limiting.

There are two problems to be overcome:  first, you need enough signal strength to reach
the limiting threshold to have a clean signal.  This can be accomplished with an
amplifier or by using a better antenna, or both.  The second is that all the amplifiers in
the chain must generate little enough noise so that the SNR isn't degraded:  receiver
noise is more complex and isn't simply AM.  It masks the desired signal, and noise
contributed at the first amplifier stage gets amplified by all successive stages along
with the desired signal.  Adding an amplifier stage does NOT improve the SNR if the
noise figure is worst than that of the original receiver (unless the coax losses are
significant.)

I've demonstrated this by cascading two preamps on a weak digital TV station:  the
receive signal strength indicator went up when I used both preamps instead of one,
indicating that the received signal was stronger, but because the SNR stayed the
same (both had about the same noise figure) there was no improvement in the
demodulated signal.


Many of the problems encountered with adding external wideband preamps are due
to interference from other stations, etc., because the preamp typically increases the
strength of ALL signals from 30 to 1000 MHz, and some designs can be easily
overloaded (or the added gain can cause overload in later stages.)


« Last Edit: November 23, 2012, 11:11:26 AM by WB6BYU » Logged
AC5UP
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Posts: 3822




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« Reply #9 on: November 23, 2012, 11:45:49 AM »

I don't think I was the one who made that comment.  It's too brief and concise. Grin

Yup... My Bad.
Added the quote as an afterthought to reduce any confusion about what I was responding to then went too far up the thread for the call sign.

I've demonstrated this by cascading two preamps on a weak digital TV station:  the receive signal strength indicator went up when I used both preamps instead of one, indicating that the received signal was stronger, but because the SNR stayed the
same (both had about the same noise figure) there was no improvement in the demodulated signal.

That's a good point, as I was an early adopter of over-the-air DTV and have observed this as well...

Signal strength is a good thing when the s/n ratio is also good, otherwise you'll see other issues. I have one station that won't un-squelch without the drop amp but that's not guaranteed from day to day. Whenever there are thunderstorms in the area I can watch it freeze or stutter with every static crash and on windy days the tree in the front yard does the same. More in the summer than winter, and I have another station that's plagued by multipath which seems to be more prevalent when the roof is wet.

In some ways NTSC was less finnicky and more reliable than DTV, but when the digital is working as it should the picture quality is fantastic. No chroma delay, no noise in solid colors, and fantastic grey scale rendition. On the flip side DTV is prone to jaggies and there was a time early on when lip sync was a challenge. But aside from that, we're not going back...........

BTW:  I have an attic antenna that's split for both TV and FM so in my case a wideband drop amp is a good thing.
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VY1MR
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Posts: 8




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« Reply #10 on: December 02, 2012, 03:04:47 AM »

Thanks for all the top shelf information OM.
After review and consideration of your adivsments
I have decided to take to following actions:

    - Build a three element Yagi
    - Set it for vertical polarization
    - Get the antenna up higher
    - Build a band pass filter into the preamp

I'm still going to build the preamp that I linked to because I had already
ordered the parts before I posted my question. 
Thanks again for all your help
   
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WB6BYU
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« Reply #11 on: December 02, 2012, 07:44:35 AM »

Quote from: VY1MR

    - Build a three element Yagi



That's a good start.  I'd suggest something like the designs of WA5VJD:

http://wa5vjb.com/yagi-pdf/cheapyagi.pdf

These are easy to build and don't require a balun.  They are mechanically
more rigid than those with a split driven element.  I've used this approach
for a number of yagis from 121 MHz to 732 MHz with good results.

I use #8 aluminum "ground wire", but I'm not sure it will hold up in a Yukon
winter.    It's cheap and easy to build a test antenna, however, to allow you
to get an idea of whether 3 elements will be sufficient for your needs, then
you know it is worthwhile building a more rugged version.


Quote

    - Set it for vertical polarization



This depends on the antenna orientation at the transmitter.  In the US most
FM BC stations broadcast a mix of vertical and horizontal (the latter to allow
reception using whip antennas on vehicles).  If that is the case for your
station, horizontal polarization may work better, but if there are reflections on
the path then you may have to try different antenna polarizations and see
which works best at your location.  (A circularly polarized antenna might
help, too, but linear polarization is generally easier to build.)


Quote

    - Get the antenna up higher


Very important.  There often is a trade-off between antenna height and
the size of antenna that can be supported at that height:  you may find
that something like a diamond quad loop reduces wind loading and allows
for a taller structure, but that is something that you have to work on
given the materials you have available.


Good luck!
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