Lets talk antenna for an 80

[rhyary]

I moved the alarm indicator over to the left side of the steering wheel where there are more blanks. The wire behind the dash was long enough so that no rewiring was needed.

More blanks? Marco already said that joke...
http://www.risingsun4x4club.org/forum2/showpost.php?p=159335&postcount=97

 

[60wag]

Hmmm.. well you still have options: toss the alarm indicator inside the dash somewhere because you don't really need to see it, or get out the jigsaw and start carving rectangular holes in the dash

 

[rhyary]

I thought about it, but on the LX450 there is a sensor located next to the blinking red.
I think it is for inside temp for the auto climate control.
I didnt want to mess with it further. it has been already moved from up row to low row with unknown consequences. It may be the cause for 12mpg?
Just kidding.

 

[60wag]

I think the sensor is the glass breakage sensor to trigger the alarm.

 

[corsair23]

I think the sensor is the glass breakage sensor to trigger the alarm.

Yep, that be what it is and whether it even works?

Rami, the sensor for the HVAC that you are probably thinking about I believe hides somewhere behind the little "vents" below the fan off button. IIRC there is a white tube stuck to the back of the vents that goes off somewhere...

 

[rhyary]

Rami, the sensor for the HVAC that you are probably thinking about I believe hides somewhere behind the little "vents" below the fan off button. IIRC there is a white tube stuck to the back of the vents that goes off somewhere...

Oh yea, I think you are right. I porgot bout that one.

 

[Inukshuk]

Revival -

Current Antenna: Larsen NMO 150 5/8 wave ground plane antenna for trail and a shorty kept handy in the rig.

An Elmer member to be suggested a "no ground plane antenna". Maybe, but:

Will the expanded metal floor of a roofrack function as an effective groundplane? If not, and if I wish to avoid drilling the roof for a mount, which BTW would then cause the antenna to be sitting low relative to the rack, would a metal disk, probably stainless sheet, make sense to do? Could the disk be effective if it was on the underside of the expanded metal or must it be on top? What size? My reading suggests 18" diameter.

Also, the expanded metal is heavily powdercoated. Does the mount need to have clean metal contact to the expanded metal - which would perhaps be another reason to just locate a disk up there.

OR, should I just clamp it to a crossbar and call it good.

I also plan to move the CB antenna up there.

Will metal fuel cans interfere?

I assume non-metallic loads will not (ie, firewood, plastic gear boxes, etc.)

Thanks!

 

[Inukshuk]

An antenna in the middle of that expanded metal floor would be about as perfect a mobile VHF or UHF as you're likely to find!

Dave, you rock.

....Radio waves will find anything conductive for their return path.... It's your job to give the energy an efficient way to get home to the radio

Doesn't the coaxial provide the return path to the radio?

Just need to remove powdercoat enough to make contact at one spot.... If it was electrically isolated at the feedpoint, that would be a problem....

What is a "feedpoint"

The contact at the one point, is it for the antenna bracket, which then provides the ground plane connection to the antenna?

CB: Same principles apply, although the size of the ground plane grows from 18" to about 8 feet. Everyone runs a compromised ground for CB (and HF ham for that matter). Our best hope is that the whole car acts reasonably like a ground plane. The mount for the CB antenna needs to conduct to the rack and it would be best if the rack itself was also conductive to the truck body. See the thread on 'bonding' that KD2KDK started when doing his HF radio installation.

Would the same ground wires used for the lights work for the CB ground, or must I run the lights on a ground wire back to the battery and separately ground the rack to the roof?

Thanks!

 

[Seldom Seen]

OR, should I just clamp it to a crossbar and call it good.

Yep, get a roof rack mount and make sure you have a good electrical connection, then switch the NMO 150 for a 1/2 wave (something like a MNO 2/70) and call it good. You have plenty of metal under the antenna to dissipate any common mode current (is that the correct term, Dave?) keeping it off the feed line.

Don't get hung up on "ground planes" unless your running a ground plane antenna ie: 1/4 wave whip. A half wave is a "no ground plane antenna" but reality it's a ground "lessdependent" antenna, no ground plane required you just need to keep the feed line from radiating.

For the Chicken Band antenna, use a couple of ground STRAPS not wire. They'll be good for RF and DC.

 

[Inukshuk]

Yep, get a roof rack mount and make sure you have a good electrical connection, then switch the NMO 150 for a 1/2 wave (something like a MNO 2/70) and call it good. You have plenty of metal under the antenna to dissipate any common mode current (is that the correct term, Dave?) keeping it off the feed line.

Don't get hung up on "ground planes" unless your running a ground plane antenna ie: 1/4 wave whip. A half wave is a "no ground plane antenna" but reality it's a ground "lessdependent" antenna, no ground plane required you just need to keep the feed line from radiating.

For the Chicken Band antenna, use a couple of ground STRAPS not wire. They'll be good for RF and DC.

Sean is also advocating the "no ground plane" antenna. Either way it looks like I'll want a rack to body ground strap.

 

[Inukshuk]

Being ground plane independent is a misnomer, an end fed 1/2λ antenna still needs a return path. Being 1/2λ just means the antenna is self resonant and does not need the counterpoise to radiate RF.

The feedpoint impedance will be VERY high and can develop a pretty sizable voltage (at 50W it can be 250 to 500V pretty easily, the theoretical feedpoint impedance is infinite and practically ends up >1800 ohms). This is a particular problem if you don't give the antenna a balanced feed, i.e. when you use coax and mount it with zero ground plane. It is critical that the antenna mount be grounded. This is why Larsen recommends an NMO mounted in the roof of your car or artificial ground radials. Trust me, you can get a pretty good tickle from outside of an uninsulated floating 1/2λ feedpoint...

BTW, this is not common mode current per say, but a byproduct of the actual differential counterpoise currents (consider it the 3rd path on the outside of the shield, you will have current on the center conductor and the inside of the outside conductor). So choking the shield is a band-aid for antenna inefficiency. OTOH, without a choke the coax will be radiating, which is never good.

This is the problem with unbalanced feedlines, resonant antenna currents are always differential and there has to be a conversion somewhere if you feed with unbalanced cable (coax). If you don't provide it, RF will figure it out for you (it finds the assumed capacitively coupled return) but it might not be ideal. Antenna manufacturers know this limitation and build in LC tanks to couple the feedpoint (transform impedances) to the unbalanced feedline, but that circuit is assumed to be referenced to RF ground. It should be mentioned that it's almost impossible to have a 100% balanced system, so chokes are absolutely acceptable to minimize common mode currents on coax. Just key to understand that choked currents are lost to heat and are not doing any work for you.

Thus, this is why people drill NMO holes or bolt directly to the body of the truck. IOW, if you are mounting the antenna electrical isolated from your RF ground the advice to use braid to ground the outer ring of the mount is very good indeed.

Great advice. I just don't have the brainpower to understand it.....