VehiGAZ

Thanks for piping in, Doc! What does "it's a DC block for the input stage of the receiver" mean? Is it keeping DC voltage out of the headunit or is it blocking DC-induced interference out of the antenna signal?

My main concern remains unanswered: Will I get good reception without the cap in-line?

The OEM antenna has already been replaced with a shark-fin antenna, and the head-unit is new - the only original component is the coax antenna wire from the HU to the antenna. For testing purposes, I will pop 12 VDC on the OEM antenna and plug its antenna pigtail directly into the HU to see if that restores reception. If it does, that means that I should be able to skip the cap, pop a new male coax connector on the end of the OEM antenna wire, and be done with it.

WallyP

"PS - Wally, nice thinly-veiled crack on my respect for Alan's electronics expertise! Despite the jab,..."

Huh? No jab, no crack. As I have shown repeatedly, I have a great deal of respect for Alan's expertise, and for his willingness to try to explain electronics to us non-boffins. Why would I make a crack if you have similar respect?

VehiGAZ

Sorry then - I thought that the "I'm not a boffin..." line was a poke - a friendly one - at Alan. No worries, we're all just having fun here!

Maleficio

Excellent thread. I've never heard of cap in an antenna lead, though.

dr bob

I vote for L-C options for correcting mismatched antenna characteristics.

[choke-choke]


A small choke in series would be used to make a short antenna look longer to the radio. A cap would need to be in parallel. A choke physically ooks a lot like a resistor, and has colored bands indicating value similar to those other R and C pieces. Check with a DMM in diode test mode to see if there's DC continuity through your part. A choke will pass DC, resistor obviously will pass some, non-polarized capacitor passes none.


----

I used inductive reasoning...

Bob Rouleau

Goodness Henry, choke - inductive reasoning, puns by the handfull

dr bob

Just checking to see who was really listening.

LT Texan

I've never seen a cap or resistor (or choke) on an auto antenna lead.

Why don't you disconnet the lead and see if you can measure resistance of with a DMM? I can't imagine why you would want annenuation of the antenna signal.

If it is a cap or choke, I'd guess that would give you a high pass or low pass filter.

But if your new antenna instructions do not recommend this component, why not remove it?

Heck, your reception is already bad with it as is.

VehiGAZ

Well, here it is folks... as you can see, the connection between this plug and the coax wire was not in good shape. I am pretty sure that plastic tube insulating the center filament was broken near the connector, shorting the signal. Combined with the fried capacitor (you can see what I mean, not doubt), it's no wonder I have no radio reception! I hope you can all see that the cap is in-line, not across the coax leads.

The band colors appear (to me) to be brown-red-brown-gray---gold. Would that make it an 8100 pF capacitor with a +/- 1% pF tolerance?

My Grand Plan to test the system by plugging my OEM antenna directly into the headunit (eliminating the cap from the circuit) came to naught, as I could not find the antenna base. I knew where the aerial was, but the base wasn't with it. The things you lose when you move...

auzivision

Now that's funny... surprised I had the capacitance to understand.

Couldn't resist.

VehiGAZ

That's currently the best joke on the thread!

docmirror

You asked some questions about the stages of the amp back there, but I don't want to get too far into radio design. Here's a short primer. The antenna gathers in a radio signal from the air. That signal needs to touch a high gain amplifier first thing so that we can boost the incoming signal. The high gain amp must use a very low bias on the gate(or base) of the amp. That bias voltage can be easily dissipated into the antenna unless it's blocked by a device which passes high AC frequencies but blocks DC voltage. In mechanical terms, think of the cap as being a rubber torque coupling between a two stroke engine, and the driveshaft to the prop. It lets the torque through, but blocks the vibration.

Now you mentioned the shark fin antenna. Remember that I said antennas are usually fractional wave. Well, one of the most important rules of antenna reception is that the lowest fraction multiple, gathers the best signal. Of course, it would be foolish to drive around with a 25ft FM antenna on a car, but the new 'modern' antennas are higher fractional numbers of the primary wave, and that captures less of the signal from distant stations. So, not to be droll, but in antenna terms, bigger really is better. A 1/4 wave antenna is about 35% better at reception than an 1/8 wave, and so on. Also, antenna that are painted lose a bit of efficiency, and antennas which have a curved profile also loose a bit, but the actual element in a shark fin is prolly straight.

VehiGAZ

Bigger may be better, but a shark needs a fin!

J/K... I put the shark fin antenna on because it made it easier to put a car cover on. Plus it looks cool, and satisfies a man's need to mod.

And I listen to only one radio station - the local NPR station - and it is one of the stronger signals in the area, so I don't care about distant stations - I just want to hear the news on the way to work!

Thanks for the antenna primer!! I'm learning more and more about it, but I've still got a long way to go.

docmirror

No prob. If there's anymore detail though, we'll have to break out the books, cause I've about covered everything on primary stage of the radio. Glad you got it working suitably.

John Speake

Looks like a 180 ohm resistor...