FM Antenna....

[JBL GUY]

Quote:

Originally Posted by JBL GUY View Post
Some people have had satisfactory performance from the BIC Beam Boxes.

SOTA

I love my FM 10, really good antenna. Ever see inside one?...

Yes, I am quite familiar with it's construction.

It uses 4 1/8 wave elements and a switching network to achieve directionality and an adjustable matching/tuning network.

It's directionality can be useful for reducing multipath interference, although with only 4 directional positions, putting it in a wooden lazy Susan, to fine tune the directionality is not a bad idea.

When I was doing a lot of traveling, I designed and built both 2 meter (146 MHz) and 440 mHz versions as portable travel antennae just for a project and something to do.



I have actually taken the FM 8 and FM 10 versions to the antenna test range and compared them to a reference dipole and a reference 1/4 wave ground plane antennae.


Note, I did not say it is a bad antenna. It has it's applications and as I have said many times, when it comes to antennae, it is location, location, location when it comes to choosing an antenna.

 

[knockbill]

I roll my own...

Do you use a lightning aresstor ? Seems it could attract more than FM!!!!!

 

[knockbill]

Lots of good stuff,,, I guess I'm lucky, the di-pole works well for me... and I thought I might find something smaller!!

 

[SOTA]

I roll my own...

Do you use a lightning aresstor ? Seems it could attract more than FM!!!!!

If you look at my 4th pic you will see that there is a ground lug at the base. From there it can go to a water pipe, ground rod etc.



J.R.

 

[knockbill]

If you look at my 4th pic you will see that there is a ground lug at the base. From there it can go to a water pipe, ground rod etc.



J.R.

I did see it,, but, you still need an aresstor between the antenna and ground, don't you? If you just ground teh antenna, it won't deliver teh signal, or is that just AM, that has a separate ground and antenna?

 

[SOTA]

Grounding the stub has shown no degradation of the signal being received, at least in my case. I have read numerous debates both pro and con on various ham radio boards but in our cases we're not using the j-pole to transmit any signal so maybe "looser rules" apply for us.


J.R.

 

[knockbill]

Grounding the stub has shown no degradation of the signal being received, at least in my case. I have read numerous debates both pro and con on various ham radio boards but in our cases we're not using the j-pole to transmit any signal so maybe "looser rules" apply for us.


J.R.

OK,,, I was just concerned with a lightning hit...

 

[JBL GUY]

In general, if an antenna takes a direct lightning hit, all bets are off.


knockbill

I did see it,, but, you still need an aresstor between the antenna and ground, don't you? If you just ground teh antenna, it won't deliver teh signal, or is that just AM, that has a separate ground and antenna?

This may help you understand antennae. BTW the same applies to antennae of any frequency of operation, AM or FM.

In order for current to flow, there must be a complete circuit. The ground connection on a receiver's AM antenna connection provides a low impedance ground for the RF signal and this may increase the received AM signal strength.

The long wire AM broadcast band antenna may be at direct current ground potential and still receive signals because the radio frequency feed point impedance is not at ground potential.

In terms of RF (radio frequency) signals, there may be a difference between the direct current resistance and the radio frequency impedance of a circuit, this is why it is possible to ground the J pole antenna.

In the case of a J Pole antenna, if you look close you will see that it is basically a continuous piece of pipe with right angle connectors, one continuous conductor.

In terms of DC resistance, all of the antenna is at ground potential and if one measures the direct current resistance at the point where the feed line is connected, it will measure right at 0 Ohms.

This may provide some protection from lightning, if the antenna is correctly grounded.

The RF impedance at FM broadcast band frequencies at the feed line connection point should be close to 75 Ohms.

BTW, the operation of a J Pole antenna may benefit from the use of a choke balun, not to match impedances, but to decouple the antenna from the feed line.

The J Pole antenna, depending on design, may have some gain relative to a simple dipole, and it can be mounded horizontally, to achieve a bi-directional reception pattern.

 

[K7sparky]

A KLM 17 element 2M LPY mounted vertical polarized at 100' is tuned abt 50MHz too high but really kicks ass for rx DX FM station reception.

Paint the copper j-pole any way you want to. Won't make a bit of difference. It is a GREAT antenna rx or tx.

There is nothing better than an outside antenna if there is any way you can do it.

Can you sneak some RG6 or 59 from your setup outside? The Bow Tie antennas work great also. You may need a 300 ohm to 75 ohm ballun but they are cheap at full price and thrift store available for pennies. The cheapie "T" (folded dipole) you mentioned with a 300 / 75 ballun mounted vertical outside and fed inside with coax will really surprise you with how well it works.

 

[Anti_Skate]

This is what I have on the roof. Pretty decent quality for the money.
http://www.mcmelectronics.com/product/30-855

 

[K7sparky]

This is what I have on the roof. Pretty decent quality for the money.
http://www.mcmelectronics.com/product/30-855

That's what I called a Bow Tie and I don't know where I got that name. I've got an OOOOOOld one side mounted offset about 3' at about 40' and it is the one I use 99.9% of the time for FM. I had to replace the little piece of 300 ohm twin lead / phasing jumper between elements after the insulation rotted off the original. Put a 300 to 75 ohm TV ballun on it and ran RG6 to it.

I live way out in the country and that antenna works like Zorro's sword. Doesn't even need a LNA and feeds 2 to 5 receivers through TV splitters that don't have FM rejection notches (vary few have notch filters and they always cost MORE and maybe not still available).

Another comment on the J-Pole. If it is going outside, sealing the end of the coax where you split it to hook to the antenna is a major problem. I have tried all types of goo, tape, air dry varnish over tape and all seem let water through after some years. Water migrated into coax = junk :thumbsdn: The only solution I have found to work well is to use a chassis mount connector for the leads to the J-pole and terminate the coax in a proper connector. A little silicon grease inside the connector, coax seal or HVAC sticky around the whole thing and do a good tape job and it lasts till the cows come home. The pictures of how SOTA did it on page 1 is as good as it gets!