Question for the guru's

[JCH]

I have a toyota tacoma, the "truck" has a plastic/composite bed. I've had a 102" whip with spring mounted to the bed just behind the truck cab, bonded the metal quarter panels to the frame, and also grounded the mount with braid. SWR was low enough, 1.3 to 1.5-ish, and it seemed to tx good. Rx, however, was wierd, could not hear weak signals on AM, could only hear one side of a local conversation which is wierd on AM since I'm listening to trucker traffic, and was not hearing two guys very well only 4 miles away the other day. I thought my 2950 should be doing better especially since the Noise levels have been super low for the last month or so.

Anyway, my question is, how can the antenna have a good swr and not receive worth a crap? I put a wilson 1K on the roof, mag mount, and it blows the whip away on rx. This is on AM.

The whip seems to receive and tx OK on SSB, as far as i know.

Thanks for any info. All I've evaluated my system with is an SWR meter, I don't have an analyzer.

 

[W5LZ]

SWR only tells you things about how well the impedance match between antenna and the rest of the system is. It can't tell you anything about how well you will hear a signal, or if it's even possible to hear it at all. That is more or less up to the capabilities of the antenna being used an how/where it's mounted, and a gob of other things. I have heard signals on an antenna with terrible SWR that I couldn't hear with an antenna with a 1:1 SWR. Using SWR as a guide to how well an antenna is doing is not a very smart thing to do.
- 'Doc

 

[JCH]

Thanks doc. I listen more than anything so I haven't messed up my radio. I thought I had my problems solved but the vehicle lacks sufficient ground plane for where I had the whip mounted I guess. I thought that lack of ground plane would show in the SWR though - but your post above answers that question.

 

[W5LZ]

This Groundplane thingy is reaching mystical proportions. It seems to be an elusive magical thing, when it's far from being anywhere close to that. That 'groundplane' is simply the 'other half' of the antenna. It usually takes some random form instead of a symetrical one, so is harder to predict with anything close to accuracy.
What does that 'other half' of the antenna consist of? Basically it's the metal structure the antenna is mounted on, and because we're talking HF, it also includes the 'dirt' the vehicle is sitting over. That 'other half' of the antenna has to have the same electrical characteristics as the 'real-live' antenna half. That means that whatever is included in that 'other half' has the same electrical characteristics as the antenna so that the whole combination is a tuned antenna. Well, because a mobile antenna on a vehcile depends on where that vehicle is, what it's over, how easily that 'dirt' is 'connected' to the vehicle, that part is almost always a variable. Move that vehicle 10 feet and things have changed to some extent. The practical outcome of it is that the antenna is always going to be a compromise due to circumstances. Which means that expecting to get and -keep- a 'perfect' 'groundplane' is as 'iffy' as winning the lottery. For most of us, getting even close is a 'win' situation.
So what are those electrical characteristics that make for a good 'groundplane' thingy? Oh jeeze, now you get into the electrical 'theory' thingy, and that means some fairly heavy-duty 'learning'. It sort of boild down to getting rid of unwanted reactances. Those reactance thingys are good stuff in the right places, but terrible in the wrong places. And there are two kinds of reactance, '+' inductive, and '-' capacitive. If you have the right amount of both kinds the combination equals 'zero', or none. It gets deeper, find your mathematical 'life-preserver'! You'd also better find a good teacher for it, cuz' I ain't one.
Every bit of this is describable in mathematical terms, or in terms of physics. It's really not magic at all. It may certainly seem like it to us what ain't exactly up to speed with it, but that's our fault.
This sh** can be soooo much fun... after it starts making sense. Before then, it's all magic. Why the hell can't you just stroll into 'Mordur' (or however you spell it)??
- 'Doc

and I ain't no guru...

 

[JCH]

I hear you. I'll read the other antenna threads for the what for and why is, my setup is supposed to work it just puzzled me is all when it won't. Yes, this radio shtuff get's a little too complicate sometimes.

 

[midnight special]

i has the same problem with 102 whip on my pontiac,,, turned out the coax i had on there went south and even though it showed 1.2 swr it wasnt right replace coax with better amphenol and throwed radio shack stuff away it improved recieve and even got out better,,, the bad coax was acting like a dummy load....

 

[w9cll]

Have a read here. Its mostly Ham related but everything applies to 11 meters as well.

 

[Robb]

i has the same problem with 102 whip on my pontiac,,, turned out the coax i had on there went south and even though it showed 1.2 swr it wasnt right replace coax with better amphenol and throwed radio shack stuff away it improved recieve and even got out better,,, the bad coax was acting like a dummy load....

The best place for Radio Shack/Tandy cable is in the garbage. If you have ever taken it apart, you will have found practically no shielding. Thinnest braid possible.

I used it once to tow a car a couple of miles, and it did OK for that. Five or six loops between both car bumpers and it worked. Sure glad it was a small car that we towed though - lol!

 

[JCH]

Thanks guys. I think its the plastic bed on the truck. Going to roof mount as soon as the parts get here.

 

[Captain Kilowatt]

The best place for Radio Shack/Tandy cable is in the garbage. If you have ever taken it apart, you will have found practically no shielding. Thinnest braid possible.

I used it once to tow a car a couple of miles, and it did OK for that. Five or six loops between both car bumpers and it worked. Sure glad it was a small car that we towed though - lol!

They must sell all the good stuff up here then. The coax R/S sells is not the best but it is not nearly as bad as a lot of people say it is. The old stuff that was actually made by Tandy Wire and Cable was actually quite good. Over the years the quality has declined a bit. The denizens on QRZ mostly parrot something they heard without actually knowing anything different.I haven't bought any R/S cable in the last several years however the quality complaints have been around for much longer than that. The last cable I bought was a lot of three full 500 foot rolls of RG8X mini foam I bought on a close out special of five cents per foot. Too cheap to pass up. The outer sheath is fine and the braid is nearly full coverage. If you flex it you can see the inner foam a bit so I suppose it is about 90-95% coverage. I have used it a LOT for jumpers and patch cables and replacing cables on mobile antennas that have deteriorated without issue. I have also used it to make 1/2 wave length tuned feeders for making precise antenna adjustments. Is it as good as Belden or Times? No. Is it as bad as most people say? No again. Would I run legal limit through it? Three for three says NO. I will stick with my Andrew LDF4 heliax and Belden 8214 for that.

 

[Mudfoot]

I've had good luck with RS RG-58. Their RG8, not so much. Most of it was concentric. I was a heavy user of their cable back when.

 

[ghz24]

RE: http://www.k0bg.com

Its mostly Ham related but everything applies to 11 meters as well.

Not every thing.
The following statement is not factual for 10 -12 meter where it is possible to get a 1/4 wave on a vehicle. And this is a CB question.

Unadulterated fact: If your HF installation doesn't require any matching (SWR under 1.6:1), you either need a better antenna, a better mounting scheme, or both!

I'm surre it's true on 40 meters and lower frequencies.
The original article even said with the exception of 10 and 12 meters.
He didn't include 11 meters because he's a ham

 

[W5LZ]

C.K.,
I can remember when coax from Radio Shack was much better than it is now. That goes back several years (several bull, it was like 20 - 25 years ago). I've resorted to R.S. in the last 4 years or so and was really disappointed. It worked, but no where near the same 'quality'. Shielding was very poor, much less than 95%. (RG-58) I'm still using one piece of it, mainly because I'm too lazy to reroute the longer run of better stuff. If the quality up there isn't better than thiss stuff I'd be surprised. The local store only carries RG-58 now, nothing else. I figure when they run out there won't be any coax at all.
-'Doc


(You know, I've been tempted to call you Captain 'K' but I'm afraid it'd confuse you with Captain Kangaroo and I don't wanna do that.)

 

[northern35s]

RE: KØBG.COM


Not every thing.
The following statement is not factual for 10 -12 meter where it is possible to get a 1/4 wave on a vehicle. And this is a CB question.

I'm surre it's true on 40 meters and lower frequencies.
The original article even said with the exception of 10 and 12 meters.
He didn't include 11 meters because he's a ham

I don't believe the statement to be incorrect, especially when he goes on to clarify:

And, to repeat, if your HF antenna doesn't require matching to provide a low SWR (with the possible exception of 10 and 12 meters), then you need a better antenna, a better mounting position, or both!

K0BG doesn't talk about CB, he has no interest in CB (that I know of), however there is nothing wrong with his statement, he even said with the possible exception of 10/12m. If we take it a step further, then a 1/4WL antenna has around 35 ohm impedance, not the 50 ohm our rigs are designed for, this adds even more credence to the statement above.

There is one caveat with reference to CB, a number of base loaded CB antennas, like the Wilson and the Sirio, already have a shunt feed arrangement in their base, these antennas have the very matching device that Alan is referring to, if they don't, like the simple wire wound antennas; Firestick, Modulator et al, then it is loss in the antenna system that produces a good 50 ohm match, the very reason why Alan suggests getting a better antenna

 

[Robb]

Whoa . . . . Look at this . . . A piece of Radio Shack RG-8 coax from the late 70's with the cover peeled off . . . My, my, my . . .

I used a LED flashlight to get more detail. As one can easily see, that is not what one would call 'shielding/braid'. Better to call it 'lack of shielding'. Unbelievable.

Can you imagine using this stuff for 2m???? No foil and almost no braid = No chance. The spaces between the braid are much wider than the braid itself. Wouldn't even use it for the HF band, although it would barely pass for usable there at those freqs. Losses would be considerable @ 100 ft compared to just about any other coax.

This would be an example of really poor quality coax . . . Pure junk . . .