| bm_mode |
Just wanting to cheaply/quickly/easily boost the pulling power of my tuner. Are those cheap RF amplifiers much use to boost RF reception any good?
Also, anyone know of a useful link for a DIY FM antenna and other ways to optimise reception ? (I'm not keen to modify the circuitry inside my tuner - the tuner's good, just the location is in a RF hole.
Cheers. |
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| Jax |
A booster or antenna amplifier only helps if it has lower noise figure than the input stage in your radio. No booster beats a good antenna.
I found a link that looks good:
http://radio.meteor.free.fr/us/yagi_fm.html |
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| dhaen |
Jax is correct when the booster amp is fitted at near the tuner.
If the booster is fitted at the mast-head (next to the antenna), there will be an improvement. This is because the loss of the downlead is subtracted from the noise. There is also sometimes an improvement in VSWR (matching).
At 100MHz 'ish the improvement will only be significant if the downlead is poor quality or very long.
You have got a roof antenna....:rolleyes: ? |
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| sreten |
I've always been under the impression that an amplifier cannot
help a good good tuner except for exceptionally bad reception
conditions and a very good antenna is still needed, and a mast
head amplifier is the way to do it.
Or the device guaranteed to make other FM enthusiasts
green with envy, the remote mast head rotator.
My one book dealing with antenna's mentions circular polarisation,
and has a design for a freestanding vertical end fed half wave
antenna, its the most ergonomic indoor design I've seen.
(Basically a base with two 1/2" round vertical
sections, one 7.5ft long the other 2.5ft long)
What antenna are you using at the moment ?
:) sreten. |
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| Jax |
| quote: | Originally posted by dhaen
If the booster is fitted at the mast-head (next to the antenna), there will be an improvement. This is because the loss of the downlead is subtracted from the noise. |
Yes, that is correct but a good antenna still beats a masthead booster ;)
It does not hurt to have both provided the quality of the masthead amp is good enough, not that wideband junk. |
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| Elso Kwak |
Hi, the best booster is a multi-element antenna. The more elements the better. In the UK is a firm advertising in a UK Hifi journal that has a nice one. Sorry don't recall the names.
Use double shielded coax cable from antenna to receiver, as short as posssible in length to minimise losses.:cool: |
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| bm_mode |
Hi,
Thanks for your input everyone who posted a reply. Much appreciated and exactly the info I was after. I think I'll have a go at building the antenna from the link that Scott Wurcer posted. Looks simple and cheap.
I'll post another reply to let you know how well it works once I've got around to building it. |
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| Damon Hill |
I've just built and installed a jpole as per one of the
examples, with a coax feed. My ham radio neighbor loaned
me his swr analyzer so I may be able to see if it's even
remotely tuned correctly for the FM band. I cut the elements
a little long so I'll have some room for tweaking.
I'm hoping the bandwidth is reasonably wide; I used 1/2"
copper tubing (gave me some practice soldering copper
tubing, too), though I might consider 3/4" on the next one.
Relatively easy project, and the results seem good for local
reception on the stations I wanted. Lots of multipath in
my location and a few stations seem to suffer as a result
with this presumably omnidirectional antenna, but thankfully
not the ones I wanted.
My first RF project in many years! Photos may eventually
follow (I wanna digital camera but can't afford a Canon Rebel)
--Damon |
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| planet10 |
| quote: | Originally posted by c.d.s.
I have had excellent results with J-pole antenna's. |
I have one of those in my project queue... i'll mount it up on the tin roof, which will give it a good ground plane & more height. The little dipole i'm currently using does surprisingly good, even thou it is under that same tin roof.
dave |
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| jackinnj |
| both the ARRL and RSGB have excellent antenna designs in their handbooks -- in particular, the ARRL Antenna Handbook comes with an CD-ROM which allows you to tailor the gain and response to your heart's content. |
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| Damon Hill |
My jpole measured 1:1 at 88MHz and 2:1 at 92MHz at 50 ohm impedance; that's reasonable bandwidth. I may trim it to the middle of the band and try another one with 3/4" tubing to see if the bandwidth can be increased.
I've done this project 'by the book' so far and don't
understand how the antenna actually works; it should be possible
to tune it for a 75 ohm impedance though for receiving purposes
this isn't critical. The actual impedance reading was more around
40 ohms, but I'm not familiar how this instrument (MFJ Enterprises,
MFJ-249) works so I may have misinterpreted something. One meter
reads SWR, the other reads impedance and the subject is a bit
complex.
As much as I'd like to have a big log periodic on a rotator, this
is a more practical answer for now. Ben Franklin would accuse me
of making a lightning rod, and I did make sure it was well-grounded.
It's a rugged and simple design, and I recommend it if you can't
get a directional antenna installed. I also ran across a modification that will turn a jpole into a kind of Yagi vertical
antenna that has some front-to-back ratio.
No serious attempts to DX, hoping to pull in a Portland station or
two. The reception is solid on the three local stations I like:
KWJZ (smooth jazz), KING (classical), and KPLU (traditional jazz and blues, NPR) Alas, no one in the region carries Hearts of Space...
Need to replace the burnt-out lamps on my tuner display so I can
read it! |
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| ALW |
| quote: | | In the UK is a firm advertising in a UK Hifi journal that has a nice one. Sorry don't recall the names |
Elso,
Are you thinking of the Ron Smith Galaxie range? They are excellent but very large.
http://members.fortunecity.com/g7nn...ts/galaxie.html
For good FM in most places you want a good, high-gain directional antenna to help eliminate multipath, which is a killer for sonic performance.
There are large benefits to be had from having excessive gain in the antenna (almost always better than a booster amp, with their potential noise and overload problems), good high-quality low-loss feeders (use double-screened satellite-grade coax) and then attenuators at the tuner end to bring the signal into the correct order for the tuner. You want the signal level to meet the full quieting sensitivity of the tuner you use.
The advantages of this approach are the attenuator improves matching, provides some isolation and attenuates the multipath pick up from the cable itself.
Andy. |
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| planet10 |
| quote: | Originally posted by Damon Hill
and KPLU (traditional jazz and blues, NPR) |
That's my station -- i listen to it 90% of the time.
dave |
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| Damon Hill |
Andy:
In my many years of playing with outdoor FM antennas,
those Rod Smith Galaxies are the most outrageous I've ever seen.
I want one. Badly.
At one time I had a JFET preselector and a switched RF
attenuator which helped my mediocre Heathkit AJ-29 plow
through local signal overload. Since I moved to Seattle,
I haven't had much opportunity or money to play that
game. For the present, this Jpole will have to do.
--Damon, who once had a stack pair of JFD's biggest log periodics
and still has a useable Finco FM-4G. Both companies are
long gone. |
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| ALW |
I knew there's be a closet antenna fetishist who'd drool over that ;)
Andy.
P.S. I often got good results with a 'SLIM JIM' folded J antenna, when fed from a suitable balun these give excellent results in a compact antenna. |
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| AuroraB |
Just to clear up a few matters for non RF inclined readers....
The J-pole and Slim Jim are vertically polarized , and more important- horizontal omni directional antennas,......... doing very good for omni directional reception.....
For weak signal reception you need a good directional antenna, like a yagi. Drawback is of course that it is directional,... and may need a rotor if you want reception in more than one direction.
A good quality masthead amplifier will do well for all types of receiving antennas.
Polarization of FM broadcast used to be horisontal, and are thus more compatible to the traditional yagi FM antenna. Receiving with opposite polarization gives what is known as "polarization loss", reducing the signal.
Qiute strange it is, though , this modern fashion of tugging home a receiver and expecting good reception without any form of antenna!
QRT de LA5** |
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| ALW |
| quote: | | A good quality masthead amplifier will do well for all types of receiving antennas. |
This is the only area I'd disagree with you on. There's no substitute (in terms of noise figure / ultimate performance) for a higher gain antenna.
Whilst there may be good masthead amps out there, most that people are likely to come across aimed at this market are noisy and exhibit poor overload / intercept performance.
In our increasingly noisy RF environment, I'd only use a masthead where absolutely necessary.
Andy. |
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| AuroraB |
Concerning the masthead amp, - I won't disagree either!
One problem though is that most modern receivers seems to be designed for living next door to the transmitter,- most of them have rather poor sensitivity. This can be remedied by a high quality masthead amp, - but again I agree with you, - most masthead amps of today are of a rather poor quality, - and the good ones are likely to be expensive, if you can find them.
Indeed, - if the tradeoff is between a larger antenna or small antenna/amplifier, the larger antenna is the the bettter way., but even with a large antenna, a masthead amp can be a good compensation for long cable runs.
Another problem, maybe not so common, is wind load on laarger antennas. In my neighbourhood, large antennas have a very short lifetime, unless being exceptionally rigid mechanically.
( I do occasionally get wind gust at 70 knots + ..........:( ) |
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| EC8010 |
The only reason for having an aerial amplifier is to overcome the losses down the cable to the receiver - and that means it should be a mast-head amplifier, probably with quite low gain (6dB). Anything else is likely to amplify muck and expose the receiver's front-end to overload and intermodulation. Nothing beats a better aerial.
Actually, one thing does. A better feeder. Most aerial cable is so leaky that it picks up as much signal as the aerial. Trouble is, unlike the aerial, it's not tuned and it's not pointed in the right direction. Use a proper low loss professional cable with a foil Rheunissen screen. |
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| ALW |
| quote: | | Actually, one thing does. A better feeder. Most aerial cable is so leaky that it picks up as much signal as the aerial. Trouble is, unlike the aerial, it's not tuned and it's not pointed in the right direction. Use a proper low loss professional cable with a foil Rheunissen screen. |
This is exactly the reason I recommend excessive antenna gain.
Feeder pickup is always a fixed, constant value (for a given installation) and always non-zero. By having excessive gain and therefore signal strength, you can attenuate at the receiver input. This brings the input signal to the correct level, whilst attenuating the fixed, unwanted, cable pickup.
Andy. |
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| Damon Hill |
I dug out my old Heathkit switched attenuator (up to 70 db)
and plugged it into my mediocre Fisher tuner. No amount of
attenuation will drive most of the local stations into the
noise. Cheap plastic case and open wiring still leaks in
a useable amount of signal. I'll have to try to rewire the
input with proper coax; originally it was 300 ohm balanced, to
which I'd added a balun and a F connector on the back panel.
I was hoping to make a guess as to how much signal that jpole
is pulling in; wonder if my ham neighbor has a RF microvoltmeter?
My presently defunct Heathkit AJ-29 was modified for coax and
that effectively eliminated leakage.
At one time I had a FM band JFET preselector, designed by Bob Cooper, I think; it actually worked fairly well in combination
with the attenuator (which originally came with the IM-57 TV
sweep generator). There are other ham tricks I haven't tried,
such as tuned cavity traps and bandpass filters, but I haven't
been in a position to need such, yet.
I found some general comments on RF amplifiers and preselectors
with a working example of a low-noise MOSFET preamp on this page:
http://www.geocities.com/toddemslie/bf981_preamp.html |
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