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Author Topic: High mobile SWR  (Read 1983 times)
N8CNJ
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« on: March 16, 2011, 08:34:39 AM »

I can't figure out how to bring down my SWR and could use some suggestions.  And a screwdriver is out of my price range at the moment LOL.

I have a 75 meter Jetstream Mobile HF Stick (I know probably my first mistake) mounted to the aluminum catwalk on my 09 Freightliner Cascadia with 18' of RG58 coax with factory molded ends running along my frame into my cab to a butt connector and then a 3' RG58 jumper from there the rest of the way to my IC-706.

I'm using a Wilson Teardrop surface mount and have run a 12 gauge ground wire from it directly to my frame.  The ground is isolated from the center lead.

The SWR meter on the radio hits its max no matter what I do with raising or lowering the whip.  I went through and made sure the coax is in good condition, everything is mounted securely, and all my coax connections are tight.

I can't get the SWR to come down and would prefer not to use a tuner in the truck.  Any ideas?  Tips?  Tricks?

Chip
KC8SHP
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KB1TXK
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« Reply #1 on: March 16, 2011, 08:56:01 AM »

Have you verified w/ another meter?
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WB2WIK
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« Reply #2 on: March 16, 2011, 09:07:39 AM »

What's a Jetstream 75m Mobile HF stick?  I didn't know Jetstream made this.

Most 75m mobile antennas are extremely sensitive to tuning and require very critical tuning once installed, to get them resonant in the band anywhere.  Assuming you don't have any opens or shorts in the feedline, your problem may be simple as the antenna is tuned outside the band and you can't tell that by using a transmitter to measure SWR.  The "SWR meter" in the Icom is terrible and a very poor way to measure anything.

With my Hustler mobile HF whips and resonators, it's very easy to have the whip set up so it's tuned completely outside the ham band, in which case SWR really will be nearly infinity until it's adjusted.  Once it is adjusted, SWR is below 2:1 only over a very narrow segment of the band, like 20-25 kHz, and then it's much higher outside that range.

Such is life with 75m mobile antennas unless they're very large.
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WB6BYU
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« Reply #3 on: March 16, 2011, 09:42:51 AM »

Quote from: KC8SHP
...mounted to the aluminum catwalk on my 09 Freightliner Cascadia...

I'm using a Wilson Teardrop surface mount and have run a 12 gauge ground wire from it directly to my frame.


Several potential problems here.

First, how much of the antenna is up in the clear above the rest of the metal structure of the vehicle?
Or, to put it another way, how much of the antenna is running close to the metal structure of the vehicle?
That has a huge effect on antenna tuning and performance.  And, to the extent that the antenna
sways in the wind, the tuning can shift with speed.  The ideal location is with the whip mounted above
any other metal, but that isn't likely with a tall antenna on a truck that is already pushing the height
limits.


How large is the catwalk?  How long is the #12 wire connecting the base of the antenna to the
frame?

A #12 wire is rather small in this application.  Not that you need to carry a lot of current, but you need
a lot of surface area to reduce the inductance and make a low-impedance path.  1" wide flat copper
strip is a better choice, and using several of them to different points on the frame will improve things
even further.


75m mobile whips can be very difficult to tune, and should have very narrow bandwidth if it is at
all efficient.  Tuning across the band in 100kHz steps you might not find any point that is better
than 5 : 1 if the resonance happens to be midway between the points you measure.  As Steve
said, even finding the band in the first place can be difficult, and the SWR at resonance for such
a short antenna (again, if it is at all efficient) may be 2 : 1 or higher.

One of the best tools for the job is an SWR analyzer that reads the sign of the reactance:  put
it RIGHT AT the feedpoint and a negative reactance means you need to lengthen the antenna.
But if there is any significant length of coax (more than a couple feet) this becomes less accurate
(though it might help you find the band at least.)

One thing you can do is to tune though the band from ,say, 3 to 5 MHz and listen for a rise in the
background noise.  A significant increase over a range of perhaps 30 to 60 kHz may mean that you've
found the resonant frequency.  I've also tied a rope to the whip and bent it towards or away from
the vehicle - if you hear a peak in the noise with the antenna pulled away it means that the
antenna is too long.  (Note that you have to stand out of the field of the antenna to take these
measurements - your body standing next to the antenna while adjusting the stinger is enough to
throw the tuning off.  I used to lay under my Volkswagen when checking the antenna so I was
out of the pattern, though that still changed the capacitance between the chassis and ground.
And I was skinnier in those days, too.)

Meanwhile a quick resistance check with a VOM should confirm whether there is a short in the
base or coax.  You can also check the performance on a different band with a different antenna
to see if there is a problem - if a quarter wave CB whip doesn't show a low SWR on 10m or 12m
then you need to start by fixing the mount.  Make sure that is working before worrying about
trying to tune the 75m antenna.
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N8CNJ
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« Reply #4 on: March 16, 2011, 10:10:05 AM »

Ok, right now I don't have the whip above the truck (I don't have a trailer at the moment either).  But the back of the truck is metal.  The reason I mounted there was because of the lack of a place to mount anywhere else.  These trucks have the wing style mirrors (west coast mirrors) and cannot be mounted to.  I have, however, just found an after market mount that will bolt up to this mirror. 

So, I'm guessing I may need to wait until I can get the new mount before continuing in order to get the whip up higher.  I'm also going to need to look into some other antennas.  I purchased this one at a hamfest because I was eager to start using my new privileges after upgrading to General last weekend.

My 10 meter antenna (separate from my CB antenna) shows a decent (1.4:1) SWR and I have talked coast to coast on it.  I also have a 2 meter antenna on the truck that works quite well.  All using the same radio.

So, any ideas on a decent mobile HF antenna for the financially challenged?
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WB2WIK
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« Reply #5 on: March 16, 2011, 10:31:39 AM »

I'd say you picked a really bad band (75m) for mobile work if you're limited on antenna options.

2m and 11m are much easier to deal with.

For HF mobile a band that can produce good results with limited antennas is 20m.  Even 40m is much less critical than 75m: It's double the frequency and a narrower band, so antenna tuning is far less critical.

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W8JX
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« Reply #6 on: March 16, 2011, 11:58:51 AM »


For HF mobile a band that can produce good results with limited antennas is 20m.  Even 40m is much less critical than 75m: It's double the frequency and a narrower band, so antenna tuning is far less critical.


It is not just that as a 80m mobile ant is little more than a dummy load with a whip and is a serious negative gain antenna. (many many db below a reference 1/4 vertical). 40m is more practical of a compromise here and while still a negative gain antenna it is considerably better than 80 mobile whip. (the closer that antenna gets in length relative to 1/4 wave length for frequency in use the more efficient it becomes) 20 does really well as does 15 too when band is open. I would focus on 40 rather than 80 here and you will get much better results.
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W3LK
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« Reply #7 on: March 16, 2011, 02:27:40 PM »

Quote
So, any ideas on a decent mobile HF antenna for the financially challenged?

Just my own opinion ...

If you want GOOD 75/80m mobile performance, you are going to have to spend money on a good antenna - and any helically-wound antenna, no matter who makes it, does not qualify. Even one on 40m is pretty pathetic. It is going to take a Hustler, at the minimum, or a Bug Catcher-style or one of the motorized (so-called screwdriver) antennas.

And mounting/location is everything. Putting HF antennas on the cabs of big rigs are a whole different world than putting them on passenger vehicles.
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W8JX
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« Reply #8 on: March 16, 2011, 05:00:40 PM »

. Even one on 40m is pretty pathetic. It is going to take a Hustler, at the minimum, or a Bug Catcher-style or one of the motorized (so-called screwdriver) antennas.

In years past when I used to do a lot of HF mobiling and used to use Ham Stick type antennas on quick disconnects on a chain bumper mount. I had very good results on 40 for a mobile (worked VK's  several morning going to work) and was a solid performer over all. 20 and 15 did even better and worked many European and Russian stations on 15. 17 does well too. 80 was much poorer and had far less luck as antenna efficiency really suffers. Anyway did it all without a screwdriver, Outback, Tarhill or other high dollar unit. Granted I had to change antennas to change bands but did not band jump much in mobile anyway.
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K0BG
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« Reply #9 on: March 17, 2011, 06:51:26 AM »

Visit my web site.
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W3LK
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« Reply #10 on: March 17, 2011, 08:14:07 AM »

. Even one on 40m is pretty pathetic. It is going to take a Hustler, at the minimum, or a Bug Catcher-style or one of the motorized (so-called screwdriver) antennas.

In years past when I used to do a lot of HF mobiling and used to use Ham Stick type antennas on quick disconnects on a chain bumper mount. I had very good results on 40 for a mobile (worked VK's  several morning going to work) and was a solid performer over all. 20 and 15 did even better and worked many European and Russian stations on 15. 17 does well too. 80 was much poorer and had far less luck as antenna efficiency really suffers. Anyway did it all without a screwdriver, Outback, Tarhill or other high dollar unit. Granted I had to change antennas to change bands but did not band jump much in mobile anyway.

The band in discussion is 75/80 - Hamstick-style antennas on that band are dummy loads, for the most part. Yea, I ran an actual HamStick on 40m with a quick-change mount, too, and I worked quite a few countries with it. That doesn't make it a good antenna. It means the guy on the other end is doing all the work to dig a crappy signal out of the noise. My signal reports from the other stations bear that out.
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K0BG
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« Reply #11 on: March 17, 2011, 10:20:25 AM »

For the sake of me, after operating mobile for over 40 years, I don't understand why almost everyone uses the ability to work DX, and fill up a log book with them, that this directly relates to how good their antenna system is. Think about this....

Under the right band conditions, it only takes a few microwatts of ERP to circle the globe. So, it shouldn't be a surprise that you could do so with about 1/2 watt ERP. Which, by the way, is about the ERP of an 80 meter hamstick.

If you do everything correctly from mounting, overall length, high Q coil, large properly mounted cap hat, and other details, you can get about 6% efficiency from a mobile antenna that you can actually drive down the street with. Even then, the ERP is still in the QRP range!

The only thing it proves, is the gullibility of far too many amateurs.
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W3LK
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« Reply #12 on: March 17, 2011, 01:04:04 PM »


The only thing it proves, is the gullibility of far too many amateurs.


<gggg>
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W8JX
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« Reply #13 on: March 17, 2011, 03:24:29 PM »

Under the right band conditions, it only takes a few microwatts of ERP to circle the globe. So, it shouldn't be a surprise that you could do so with about 1/2 watt ERP. Which, by the way, is about the ERP of an 80 meter hamstick.

Hate to disappoint you on a roll but a few microwatts o ERP and 40 is not going to get you anywhere except maybe across street. Also I have little doubt you are also guess a lot here. Never said a 80 m ham stick was efficient but would say it likely has a ERP of at least a few watts with 100 in and 40 m version would be a bit better. Granted they take a back seat to a good base antenna but I have worked a lot of 40m mobile (and never used a tuner either) and many 40m mobiles too and some have been using Ham Sticks as well. BTW, you are not going to get down to VK land on 40 SSB with a few microwatts ERP ever nor even milliwatts.

The only thing it proves, is the gullibility of far too many amateurs.

You mean the some that think going from 500 to 600 watts is going to make a big difference when it is not even 1 db. The gullible ones are one that think you need a expensive mobile antenna to be successful. 
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