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Author Topic: Remote antenna tuner?  (Read 3323 times)
WA8JNM
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Posts: 175




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« on: November 12, 2012, 03:43:38 PM »

Hello,

Here's my situation. I am in an HOA controlled environment. There is a wooded area beginning about 30 feet behind the grassy area of my condominium. The woods is about 70 foot deep.

The association has agreed to permit me to put up an 80 meter doublet, fed with ladderline, in the back of that wooded area, in the trees. Their requirements: bury the feedline. When the feedline rises to the feed point of the antenna, run it up a tree located about 20 foot from the center of the antenna, so as to minimize the aesthetic issues. Presently I'm tuning the antenna system with a good quality manual antenna tuner in the shack.

Compliance will require me to bury about 100 foot of transmission line,  from the shack to the bottom of the tree. My understanding is that burying ladderline would be a serious mistake, even if I placed it in nonmetallic drainage pipe. so, it looks like I will need to bury about a 100 feet of high quality, low loss, coax to the point where the ladderline rises up the tree trunk to the antenna. I will use a balun at the joint at the bottom of the tree, where the buried coax joins the rising ladderline.

My question is this: is it worth the cost, expense and trouble to install a remote automatic antenna tuner at the junction of the ladderline and the hundred feet of buried coax? The doublet allows operation from 80 meters through 10 meters, but I mostly focus on 40 m and 30 m.

I know there will be loss in the system, especially without the remote antenna tuner. I'm mostly a CW rag chewer, although occasionally I play with DX. I can always increase the power, if I'm not using an antenna tuner, to make up for the loss. Obviously I can't do that on receive, To make up for any loss when receiving.

What do you guys think?

Thanks,
Dave
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AA4PB
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« Reply #1 on: November 12, 2012, 04:10:19 PM »

The remote tuner would be the most efficient. The question is, is it in a protected area that will limit public access to the tuner?

The loss in LMR400DB (direct burial) should be acceptable.
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WX7G
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« Reply #2 on: November 12, 2012, 04:59:39 PM »

The input impedance on 28 MHz may be 2000 ohms. Transformed by an ideal 4:1 balun the coax is presented with a 10:1 VSWR. The VK1OD line loss calculator reports the loss of 100' of LMR400 to be 2.4 dB. How about 7 MHz where the VSWR is also 10:1? The line loss to be 1.4 dB. More exact numbers can be had knowing the length of ladder line and using NEC.


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WX7G
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« Reply #3 on: November 12, 2012, 07:49:18 PM »

I ran an EZNEC simulation of a 135' dipole 30' above average ground. The VSWR on the LMR400, given an ideal 4:1 balun at the coax-to-ladder line transition and a 90 degree, 400 ohm section of lossless ladder line is:

3.5 MHz, 16:1, loss = 1.5 dB

7.0 MHz, 32:1, loss = 2.7 dB

10.1 MHz, 7:1, loss = 1.0 dB

The big unknown here is the balun and how its non-ideal nature will affect the VSWR and consequently the coax line loss.
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NA0AA
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« Reply #4 on: November 12, 2012, 07:52:44 PM »

This is one case where a remote tuner really will pay off, IMHO.

The remote MFJ tuners even come with the power injector to power the tuner via the coax itself.

You might want to paint the enclosure a discreet camo with a small sign with your callsign and name on it.
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WA8FOZ
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« Reply #5 on: November 12, 2012, 08:18:07 PM »

Quote
My question is this: is it worth the cost, expense and trouble to install a remote automatic antenna tuner at the junction of the ladderline and the hundred feet of buried coax? The doublet allows operation from 80 meters through 10 meters, but I mostly focus on 40 m and 30 m.

Yes! The alternative would be a fan dipole, which could be much less stealthy and harder to put up in trees.
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W5WSS
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« Reply #6 on: November 13, 2012, 08:03:30 AM »

The remote auto tuner installed where you describe is a viable solution to antenna system performance.

The reliability of The various auto tuners along with lightning vulnerabilities means that some specific measures to reduce the problems should be considered.

The mains ground connection should be checked and serviced for best functionality.

A Faraday enclosure: To help protect the sensitive remote auto tuner out at that location I would use a metal enclosure and a dedicated ground rod with a wide strap connected to the enclosure and make the bond as low a resistance to the ground rod.

Now is the best time While doing so to pay very close attention to noise ingress if you include the coaxial braid of the feedline to this rod. That is not to say the lightning ground at the enclosure is wrong but one should listen carefully on each band while a helper makes and breaks the bond out at the enclosure if noise increases with the inclusion of the feedline coaxial shielding bonding via a ground connection out at this spot then there is a problem that needs further investigation.

One can opt to isolate the dedicated enclosure ground and not include the shield of the coaxial feedline of the antenna system and still be fine when the mains ground system is properly installed along with the proper techniques of connecting to it for your ham system.

Or:

Since you are not using a tower but rather a tree as the support and can not ground the tree at it's base you can opt to include a UHF barrel splice for the purpose of grounding the coaxial shield to a ground rod at the spot where the feedline meets the base of the tree.

Either choice always includes the proper function of the mains ground and your connecting the station to it.

To reiterate one can isolate the Faraday enclosure dedicated ground and not include the antenna system feedline shield since the tuner too is installed in a metallic box within the Faraday enclosure and can be installed in such a way as to be dc isolated.

A strong strike can arc and I am not suggesting it can be guaranteed avoided but it is an advantage to enclose the remote tuner at this location and isolate the enclosure from a dc continuity between the antenna feedline shield and this box.

Some distance between the shield ground and the box dedicated ground rod system is prudent where potential gradients exist with respect to a high rise around the area surrounding the enclosure by a lightning stroke as related to a shielding ground at the tree base.

High lightning strokes are problematic and often time find issues we are not aware of.




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WI4P
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« Reply #7 on: November 13, 2012, 08:32:44 AM »

Quote
My question is this: is it worth the cost, expense and trouble to install a remote automatic antenna tuner at the junction of the ladderline and the hundred feet of buried coax? The doublet allows operation from 80 meters through 10 meters, but I mostly focus on 40 m and 30 m.

Yes! The alternative would be a fan dipole, which could be much less stealthy and harder to put up in trees.

I use a fan dipole and found it no harder to erect than a single band dipole would be.  Just a little more effort required to build and tune.  It is near the shack so is fed with 8x but for your buried & longer run you would want something like LMR400.  My internal 3:1 tuner gives me good results on 80,40,20,15 & 10.

Good luck with your project and congratulations on getting the HOA's approval.

73, John
WI4P
 
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AA4PB
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« Reply #8 on: November 13, 2012, 08:52:53 AM »

"A Faraday enclosure:"

I question the value of a Faraday enclosure when there are external antenna and power connections going through the enclosure and directly to the internal circuitry of the tuner.

If you are concerned, you might add an external 12VDC relay that would short the antenna to ground and disconnect from the tuner when no power is applied.
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WX7G
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« Reply #9 on: November 13, 2012, 09:25:38 AM »

Having had two autotuners and having been unimpressed with their tuning range I would not use a remote tuner.

I would run ladder line to a Balun Designs #4115 4:1 current balun then through LMR400 to a manual tuner in the shack.

To greatly reduce the VSWR on the coax I would use an 80 meter OCF dipole.

Here's a link to an article on OCF dipoles. The author says to use an 80/20 percent feedpoint. For 3.5 MHz this is 27' from the end of a 134' dipole.
« Last Edit: November 13, 2012, 09:56:41 AM by WX7G » Logged
K8WTF
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« Reply #10 on: November 13, 2012, 10:01:10 AM »

If one were to incorporate a remote auto tuner into the original poster's design, would a 1:1 balun still be recommended between the tuner and the ladder line?  The MFJ line of remote tuners have lugs for ground and wire connections in addition to the UHF style unbalanced antenna connection.  The SGC tuners seem to have only feed and ground spades, no provisions for an unbalanced connection.

I ask because I am considering a similar design.
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W5WSS
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« Reply #11 on: November 13, 2012, 12:44:02 PM »

Aa4pb, relying on only one relay in it's own self contained housing or an added wx proof plastic housing or metallic housing,that is left ungrounded or floating and locating this 12vdc relay again away yet obviously nearby from the WX Housing for the tuner location that regardless needs to be wx proofed in a box does not fully resolve the proximity vulnerability of lightning EMP induced damaging strength voltage and current finding the inside of the tuner via the lines that regardless MUST enter the cabinetry of both layers of protection. As would be the case of a tuner located at the closest and desirable point where the feedline drops to the Earth surface.below a tree.

The automation from the operating position where we can choose to open the antenna feedline circuit of both the center conductor and shield via a 12VDC relay switch is nice but I question your suggested technique when used alone and not grounding it's housing.

Since we need to Wx proof the tuner then dedicating a grounded metallic housing is an added layer of protection that I reiterate and debate is needed even when we open the feedline circuit totally.especially in lieu of the fact that the relay switch will be located very close but as you suggest outside and not grounded as the remote tuner housing is. Why would one run a control conductor into the shack from the surface directly below the antenna? The answer is we would still need preventatives that are redundant compared to the original prescription.

As you suggest where we do not use a metallic and grounded housing for the tuner and opt with The 12VDC relay switch inherently nearby is not as effective as my suggestion sorry.
A very reliable technique to open the center conductor and shield simultaneously using a 12VDC relay switch involves all the same measures used in my suggestion in the first place. where we place the relay on the ground and still switch/connect it to a dedicated ground rod but yet do not create a ground loop with respect to the system mains ground point.is the objective yet there still remains an EMP path along the remaining shielding of the coaxial cable making it's way to the shack entry.or the control wire of the switch itself.
Not to mention that EMP is induced into the relay control line does not include whats worse actual arc across that small gap so in the end to combine several known techniches adding layers of protection is wise.

Bonding one relay switch inside the tuner Faraday housing and adding another second switch relay at the operating position or just outside in it's own self contained wx proofed housing and then opening the feedline and relay switch circuit itself in both places removes the whole line section and feedline too is probably best in every case where we want to include an remote auto tuner.
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WX7G
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« Reply #12 on: November 13, 2012, 01:50:50 PM »

If one were to incorporate a remote auto tuner into the original poster's design, would a 1:1 balun still be recommended between the tuner and the ladder line?  The MFJ line of remote tuners have lugs for ground and wire connections in addition to the UHF style unbalanced antenna connection.  The SGC tuners seem to have only feed and ground spades, no provisions for an unbalanced connection.

I ask because I am considering a similar design.

Coax can be connected to the SCG feed an ground terminals.
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N0NCO
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« Reply #13 on: November 26, 2012, 09:35:13 AM »

An efficient, wide-range L-network remote tuner would be an excellent choice for your doublet. The SGC tuners can be operated balanced or unbalanced. For balanced antennas, connect the ladder-line or radiating elements directly to the hot & ground lugs. Do not use a balun on the antenna side! That defeats much of the purpose of using a remote tuner. Isolate the tuner from ground, and install 1:1 a current choke on the feedline & power cable as close to the tuner as possible to decouple the lines from the antenna system. For unbalanced antennas, simply connect the radiator to the hot side of the tuner & and ground the cold side to your radial field with low-inductance straps.

For the record - I use an SGC 230 mounted at the base of my 160-10m inverted-L, with a 90' run of LMR 400 to the shack. I put the coax in plastic conduit to keep the burrowing critters from chewing through it. I have no problem operating anywhere from 160-10m, and I shoot DX on 160-10 with just 100W on SSB. I worked the world with that setup from a small city lot with only 60' x 60' for the antenna during the last solar minimum.

BTW - connecting coax to the antenna terminals of the SGC tuners is not recommended. It's not an issue when matching a moderate impedance, but when the mismatch is high, the resulting voltage can be high enough to arc RG8 even at lower power levels. Besides, the main benefit of using a remote tuner is eliminating the losses associated with matching a high VSWR coax-fed antenna from the shack, and eliminating the losses associated with operating a balun at a high VSWR.
« Last Edit: November 26, 2012, 10:01:40 AM by N0NCO » Logged

Joel
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