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Old 13th April 2012, 05:19 PM   #11
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Quote:
Originally Posted by DF96 View Post
For broadcast MF and HF receivers low noise is not an issue as external noise is so high, so an impedance match is more likely.
Input impedance of HF receivers is actually all over the place, rarely 50 ohms even today. Most receivers have excess gain/sensitivity which removes this requirement on signal transfer grounds.

I suppose the important question is how mismatch affects s/n ratio not absolute signal level. A 3:1 impedance mismatch might not affect it much.

In any case, as you say, for HF, front end system noise figure is dwarfed by atmospheric noise and RFI from my neighbors cheap #@$#! plasma TV!

Similarly, broadcast FM signals are likely to be quite strong, so a dB or two of mismatch loss is immaterial when the signals are full quieting anyway.

Optimizing RX antenna matching seems to be of concern only at very high frequencies and challenging weak signal operations like space comms.

In my neighborhood, a strong signal RF environment is the big problem!

Fortunately, we can listen to jazz FM quite happily with a non-optimal system.
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Old 13th April 2012, 05:30 PM   #12
DF96 is offline DF96  England
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Broadcast FM signals may be strong for some people, but certainly not all. Assuming good AGC design (not always true!) then the level required for full quieting depends on front-end noise. Of course, good handling of strong signals is needed too for those nearer transmitters so like all other electronics a compromise may be needed.

I suppose some HF receivers still have too much gain, although professional ones should be OK - hopefully they are less subject to specmanship.
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Old 13th April 2012, 08:03 PM   #13
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This probably explains why, in my experience, there isn't much difference between FM antenna designs. The important factors are height and line of sight. Not that there aren't differences, but most of us have such sub-optimal installations (say, indoors) that the antenna design never performs as it should on paper.

Right now I'm trying to improve a receiver system in a machine shop. The metal building is near other metal buildings. I can't drill a hole to the outside. There is one wall of windows I'm going to try and work with, but my guess is the physical placement of the antenna will have far more to do with the result than matching, feedline or much of anything else.
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Old 13th April 2012, 08:58 PM   #14
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For everything you need to know about FM antennas and such see:

88108 MHz
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Old 14th April 2012, 10:55 AM   #15
Mike B is offline Mike B  United Kingdom
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That www is very good but its focused on HAM radio
This www is focused only on FM radio reception - EV's Best Top Rated FM and HD Radio Antenna Guide & Reviews

If anyone is interested I have just finished a few days playing with a balun on my Ron Smith Galaxy.
The Galaxy full wave loops are close to 75ohm so its tempting not to consider a balun. But the loop is balanced & 75ohm coax isn't, so at some frequencies the coax will be part of the antenna & current will exist in the coax shield. So I set about trying a coiled coax choke to isolate the antenna from the feeder. Its easy, the formula is simple, all you need is time & an "F" coupling kit, the hard part is DIY access if its on the roof.
Its made an already good antenna even better on my Naim NAT-05, no claims of sucking in remote stations, its all about SQ & stereo.
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Old 14th April 2012, 06:04 PM   #16
DF96 is offline DF96  England
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Quote:
Originally Posted by Mike B
The Galaxy full wave loops are close to 75ohm so its tempting not to consider a balun.
Only tempting for those who, unlike you, don't know what a balun actually does. Nothing to do with impedance matching, although some baluns can do that too.

Quote:
But the loop is balanced & 75ohm coax isn't, so at some frequencies the coax will be part of the antenna & current will exist in the coax shield.
Yes, that is what baluns are for. Sadly, many commercial FM antennas omit a balun. Until digital TV came along, many TV antennas did the same. Curiously, digital TV is more susceptible to interference than analogue TV even though EEs are always taught the opposite in class!
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Old 14th April 2012, 06:08 PM   #17
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I've played with looped coax baluns a bit and that's todays project again. IMO, they're very fussy. A small change in dimensions has a huge effect on the tuning, so you either need a good recipe that's known to work, or some way to sweep or measure it. From reading the sites above, ferrites also work, but the coax balun is better.
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Old 14th April 2012, 07:26 PM   #18
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Yes...Coax baluns are a lot fussier...and there is no simple observational way to know if they working well or not.

Ferrite core baluns are better in various ways, far more predictable and well-behaved. All sorts of troublesome couplings and resonances can exist in a coax coil balun...especially if you just wind up a messy overlapping coil. And you can't know the actual performance without appropriate test gear.

If snap-on ferrite cores are used it is a simple matter to add more on for a higher choking impedance. Target should be, say, 10x feedpoint impedance so two or three appropriate snap-ons would do the trick. 300-500 ohms per core is a good ballpark estimate.

You could also get a toroidal core and wind the coax through it a number of times.

Just make sure that the cores are effective in the frequency range of interest and you should be good to go.

A dipole, yagi, or any other balanced antenna won't work as well as it should without a balun.

PS: I'd beware of F connectors for open air use...they are not waterproof and water will easily wick into a braided shield coaxial cable and ruin it. If you have to use them use waterproofing putty or self-amalgamating silicone tape to seal the connection well.
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Old 14th April 2012, 09:07 PM   #19
Mike B is offline Mike B  United Kingdom
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Quote:
Originally Posted by Conrad Hoffman View Post
I've played with looped coax baluns a bit and that's todays project again. IMO, they're very fussy. A small change in dimensions has a huge effect on the tuning, so you either need a good recipe that's known to work, or some way to sweep or measure it. From reading the sites above, ferrites also work, but the coax balun is better.
First off my thing is not a balun - yes I know that's what I called it - but it is a choke that is simply isolating the antenna from the feeder.
I researched this & found a ton of ways to "make" these coils. Ham radio www seems to be polluted with a lot of ways some very hit & miss & some way from any combo of logic/math/physics.
What I have done is to get the wire length right & make sure the coil has a self-resonance frequency (SRF) higher than the antenna/receiver operating range. If the SRF is lower thn the operating range the coil positively encourages the feed wire & antenna to work together.
The coil straight wire length should be just less than 1/4 wave length of the highest frequency the antenna is used for. 88-108MHz needs to be at or above 108MHz, this gives a wire length of 690mm (27.2 inches) The 1/4 wave wire length & the resulting number of turns sets the SRF higher than the operating frequency.

My 88-108MHz choke details:
Wire is Webro WF100 (foam dielectric & high 0.81 VF - not that VF is an issue with a coax choke)
690mm idealy has 3 turns of 73.3mm (2.88 inch) mean dia. (mean dia is the coax cable center to center) & a theoretical SRF of 119MHz.
Alternatives with this wire length - 2 turns gives 134MHz - 4 turns (too tight for the cable) gives 132MHz

Last edited by Mike B; 14th April 2012 at 09:19 PM.
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Old 14th April 2012, 10:16 PM   #20
DF96 is offline DF96  England
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Sounds like a choke balun to me, also known as a current-mode balun. In most cases better than a voltage-mode balun.

Amateur radio suffers from myths and stories (mainly to do with antennas and transmission lines), but not quite as much as audio! You might think that having to pass a technical exam to get a transmitting licence would ensure some level of technical knowledge but that is not always the case.
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