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rhyary
06-04-2010, 10:42 AM
Here is what make sense to me. Let me know what you think
I have the 4x4lab rear bumper with two swings.
The tire swing has the Hi-Lift mounting point and it is where my hi-lift is mounted.

The second swing, the Jerry can carrier, also has the hi-lift mounting bracket.

What I am thinking is fabricating a bracket that mounts to the hi-lift bracket. probably a C channel type that will go up to the roof level. On top I will have some sort of a T bracket with holes for antenna mounts.

This will give me a roof level mount that is sturdy for any antenna, and because this based will be at roof level, I can use 1/4 wave antenna, as many as I want I guess. It should be easily sturdy enough for a 5/8 wavelength of any wavelength.

Although it will be above or at the roof hight, it will not be above the truck roof. Does it matter?
Will it have less "ground plane" if it is not over the roof?

Can I make it from Aluminum?

Since the bumper is powder coated, can I used grounding strap instead of scraping the powder coat?

I looked for entry points for the coax and did not find anything that is easily used.

Although radical, I would like to drill new holes and use rubber grommet, instead of poking and disturbing existing grommets.

I am sure my issues have been discussed before, but what I want to do is fairly specific. I think.

Many thanks in advance

Rami

nakman
06-04-2010, 11:43 AM
One place to run coax is in your rear fender behind the tail light- you've already got a grommet there for the sunroof drain that is ripe for the disturbing if you swing back that direction, and plenty of room for drilling if not. But that gets the wires out of harm's way as far as the hatch and tail gate operation is concerned..

Are you installing just a 2m radio?

rhyary
06-04-2010, 12:18 PM
Yea I saw on both sides the rubber drainage hose. I suppose I can poke a hole in the grommet, like I did on the firewall grommets.

rhyary
06-04-2010, 12:22 PM
Here is the location of the bracket

rhyary
06-04-2010, 12:24 PM
In red is the bracket I am thinking about:

corsair23
06-04-2010, 01:50 PM
Rami,

You can get real fancy with fabricating the mount but I've had great success with my NMO latch mount...Thin coax goes right in the hatch and then down the interior rear panel to my rig mounted in the PS rear quarter panel area.

Here is a thread where different antenna mounts on 80's have been discussed:

http://www.risingsun4x4club.org/forum2/showthread.php?t=6434

Not trying to talk you out of your idea at all, just offering some alternatives.

As for your other questions...I'm a HAM novice so I'm sure the experts will chime in but no, your antenna doesn't have to be "above" the roof line. Dead center of the roof will give you the best ground plane but that is an impractical location for many so you go from there.

I don't see any reason you can't make your bracket out of aluminum and yes, I would suggest a grounding strap vs. scarping the powder coat. Maybe a grounding strap from the base of the antenna then down the bracket and to a good ground location on the frame.

As for an entry point for the coax. I use the gromments in the rear area that can be accessed under the rig and inside the rear quarter panels. As Tim mentioned, you already have drain tubes going out these grommets so you can usually squeeze wires, coax, etc. through the existing hole in the grommet. In my case, I have my CB coax, Hoppy harness wires, and a couple ground wires all going through my PS grommet.

rhyary
06-04-2010, 02:55 PM
Jeff,
many thanks for your insights.
My radio should arrive to day from HRO but I am going to temp everything with a cheappo mag until I figure out what I want.

DaveInDenver
06-04-2010, 03:00 PM
If you want more theory, be happy to type it up sometime. But as for your questions specifically...

The more radiating length that is above your roof line the better. The body of your truck is doing a couple of major things as far as antennas go and none of it is helped if the antenna whip is physically close to the sheet metal for relatively long lengths.

OTOH, I've mentioned this before and will repeat it, you have to come up with an antenna system that is workable and livable as much as anything because even an ideal antenna that is a pain won't be fun. Just understand that there are definite reasons that everyone says to drill a hole in the middle of the roof, that is absolutely the best location to install an antenna in all cases, but really very impractical for anything other than VHF and UHF unless you are really dedicated to mobile operating. So you do the best you can within the limits you have.

As far as using aluminum, that is fine. Make sure it's grounded just like any other conductive material since it's physically close to the antenna and will try to become part of the antenna and you don't want to be making too many stray caps and inductors.

Regarding grounding, use straps if there is any question. Unless the path is absolutely bomber it is likely that it will have some 'impedance'. Notice that I did not say 'resistance', because a fair to even good DC ground will probably not be a zero ohm impedance path, which is the combination of resistance and reactance (made up of capacitance and inductance). So a DC ohmmeter isn't necessarily going to tell you the whole story.

Other than the frame itself and large sheet metal pieces there are very few excellent RF conduction paths on your vehicle (read up about the 'skin effect' for EM fields), so ultimately what you have to do connect smaller things with braid to larger things and then make sure all the larger things are well connected. This is termed 'RF bonding' your vehicle. What you need to do is make sure a significant part has a good braid connection to its nearest large piece and then that larger piece has a couple of braids to the frame. Put a braid that bridges the swing arm to the bumper (the pivot might be OK for DC current but is terrible for AC), then a braid or two from the bumper to the frame. The only way to know that something is an excellent RF conductor is if it either has multiple continuous lengths or welds longer than say 6 or 12 inches, so if your bumper is painted and bolted it probably is a horrible RF conductor despite maybe being a great DC conductor.

wesintl
06-04-2010, 03:05 PM
or there is the famous gamiviti mout

http://www.gamiviti.com/html/products_antennamounts.html

nakman
06-04-2010, 03:11 PM
or there is the famous gamiviti mout

http://www.gamiviti.com/html/products_antennamounts.html

which are back in stock, btw :D

rhyary
06-04-2010, 03:34 PM
Dave,
Thanks for your explanation but I need to convert this to specifics:
Bare with me:

1. my bumper is connected with bolts to the frame. Is this good enough RF grounding?

2. In front, I have heavy duty grounding 2/0 and 1/0 from both of my batteries to the engine block and to the fender. These are DC grounding, but is it also good for RF grounding?

3.I also have a 2/0 grounding in from the transfer case to the frame to assist the puny ground cable the 80 has between the body and the transmission/transfer case.


4. The swing arm is on a bearings and I presume it may not be as well connected to the bumper as I would want. So I can see how a grounding strap can help.

5. Bracket I am thinking will be securely attached to the swing. Should I but a grounding strap between the bracket and the swing?

6. up on the "shelf", see my red line in the picture, I presume I will use a grounding strap between the antenna and the shelf.

To all, I have read the existing solutions and may go with those solution. But for this thread if you bear with me and help me figure out the bracket I outlined in the OP it will be super wonderful.

Rami

DaveInDenver
06-04-2010, 04:13 PM
Dave,
Thanks for your explanation but I need to convert this to specifics:
Bare with me:

1. my bumper is connected with bolts to the frame. Is this good enough RF grounding?

Without testing it you can't say absolutely, but probably not ideal. You have to picture that a capacitor is just two conductors with significant length parallel held electrically insulated by a dielectric. IOW, two painted or powdercoated chunks of steel in close proximity are pretty much exactly a capacitor.

When you put bolts (particularly smaller diameter ones) through, which are generally fairly poor RF conductors because of corrosion mostly, all you are doing is making one huge capacitor into a few just big ones wired in parallel, which is not different in magnitude than the one huge one (caps work electrically opposite to resistors and inductors, parallel is additive, series is subtractive).

What the braid does is provide a good path to alternating current (e.g. RF) that shorts the capacitor(s) and so no charge can build up, negating the bulk of the parasitic reactance. If you poked enough holes and filled them with bolts you would start to make the connection better for RF, but the ultimate goal would be to have a few tens of bolts, which would impact the connections mechanical strength and look bad to boot. You might also be able to make it work if you used unfinished stainless and aluminum with dielectric grease because the materials and bolts would make much better contact. But paint and rust and road grime all impact the path very negatively.

2. In front, I have heavy duty grounding 2/0 and 1/0 from both of my batteries to the engine block and to the fender. These are DC grounding, but is it also good for RF grounding?

Despite their size and low DC resistance, these cables are generally not good RF grounds and so it is generally good practice to put a heavy braid parallel to the cables from the battery negative to the frame and block.

3.I also have a 2/0 grounding in from the transfer case to the frame to assist the puny ground cable the 80 has between the body and the transmission/transfer case.

I would parallel that but for VHF/UHF and particularly for FM it's overkill to go crazy. But you really need to provide at least one solid RF tie for the battery negative, frame and block so that any grounds that tie into them have a good chance of making it home to the radio.

4. The swing arm is on a bearings and I presume it may not be as well connected to the bumper as I would want. So I can see how a grounding strap can help.

5. Bracket I am thinking will be securely attached to the swing. Should I but a grounding strap between the bracket and the swing?

You want a good RF path from the antenna mount to the RF ground. The antenna wants to couple to the mount and anything close to the feedpoint is lower in impedance than things farther away. If you float it relative to it's desired return path you make the system more complicated electrically. That means that you might be able to get it to tune but the more stray reactance you have the more sensitive it will be to changes like rust and weather.

6. up on the "shelf", see my red line in the picture, I presume I will use a grounding strap between the antenna and the shelf.

You want to use grounding braid between significant pieces of metal if there is any question of conductivity. What is significant changes the closer to the radiating element, too. An 8" plate under the antenna whip is very significant, while an 8" plate on your front bumper with the antenna on the back isn't critical.

wesintl
06-04-2010, 04:18 PM
To all, I have read the existing solutions and may go with those solution. But for this thread if you bear with me and help me figure out the bracket I outlined in the OP it will be super wonderful.

Rami

np.. that red line just doesn't look all that stable... :D

rhyary
06-08-2010, 08:28 AM
Dave,
Wish I could understand everything you wrote. I will at some point, but not yet.
What I have taken from it is that the grounding on my car is good for DC but not good enough for RF.

So my first step is to improve grounding for RF. Would love to have you comments:

Tin copper strap 1" by .062 from here:
http://www.gacopper.com/images/Braid-1in-Harg-1GC-Txt.jpg
and here:
http://www.gacopper.com/Braid.html

If understood you correctly:
1. I will add strap the negative of both batteries in parallel to the DC wire to fender, engine block.

2. strap the front and rear bumper to the chassis.
3. Strap the swing arm to the bumper.
4. Strap the transmission to the chassis (may be overkill)


I know I have a good DC grounding. But this will also give me a good RF grounding. Correct?

Can I use the bolts and that hold the bumpers to the chassis?
or do I need to drill holes specifically to the strap?

Looking forward to your comments

Rami

DaveInDenver
06-08-2010, 08:53 AM
That braid is fine, use the widest that is practical and easy to work with. No need to drill extra holes if existing holes line up. You don't need to swiss cheese your frame, just make sure the existing holes are clean and maybe use a bit of dielectric grease or silicone to keep them from rusting too much.

rhyary
06-08-2010, 08:58 AM
Any issue with using existing bolts that hold the bumper to the frame under the car?

DaveInDenver
06-08-2010, 09:15 AM
Any issue with using existing bolts that hold the bumper to the frame under the car?
Using existing bolts is OK as long as the braid itself has good contact to the frame or steel and you are not using the bolt as the conduction path. That's the point, to make various pieces of steel all at the same RF level. You do accomplish some of this by using DC grounds and so something is better than nothing for the most part. However all small wire grounds show some sort of odd behavior at different frequencies and even RF braid will start to have odd characteristics depending on size (width, number of wires, etc.) and quality of connections at some frequency. You can only do the best you can, so just get the easy stuff to start.

rhyary
06-08-2010, 09:25 AM
So I will have to grind the paint/powdercoat so the strap itself will make conductive contact with the metal. Hmm, rust comes to mind. Also, is it importand for the strap to be short?

DaveInDenver
06-08-2010, 09:45 AM
So I will have to grind the paint/powdercoat so the strap itself will make conductive contact with the metal. Hmm, rust comes to mind. Also, is it importand for the strap to be short?
Yup, yup and yup. The rust issue is major from both the truck and radio viewpoints (rust some place is often why a good install goes bad). You obviously don't want to go to all this trouble to make a clean install to only have your frame and body start rusting away, so you do need to balance the technical need for a good connection against ruining your truck.

As far as removing paint, scuff it to expose some bare metal and when you clamp down the braid it will hopefully contact as much as possible. This is where I use the dielectrical grease. Another option is to paint back over the connection or glop it with silicone or wipe grease on top of it periodically. I use dielectric grease under and all over the connection and periodically clean and re-do. Anti-seize with copper is a good rust preventer and stays around better than dielectric grease. There are probably paints that conduct, but IME powdercoat is pretty lousy conductor and I've gotten a mild RF burn from a poorly ground antenna that could not couple across powdercoat.

Braid, like all grounds and powers for that matter, should be long enough to make the connection without being mechanically stressed. Longer doesn't help you at all with feedlines for power or RF. There are specific reason why you'd want specific lengths, but for anything we are talking about here you want everything just long enough.

Literally everything about a mobile radio installation is a compromise, so you work with whatcha got. IMVHO you should definitely put braid in the locations on the bumper around the antennas and maybe a couple to make sure the body of the truck connects to the frame and the one from the battery to the frame. You only expose a handful of spots to corrosion and you keep it manageable.

rhyary
06-08-2010, 09:55 AM
I like anti-seize. I use it everywhere.
The reason I asked about the length is that the negative from the battery to the engine block will be close to 2 feet. otherwise, most straps can be about 8 inches.

PS, I hate compromises :-)