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Author Topic: High SWR and Short Coax  (Read 1911 times)
KE5JPP
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« on: September 23, 2012, 09:58:31 AM »

I have an inverted L that I use on 80/40/20 meters.  I had been using a SGC remote autotuner at the base, but recently it appears it has been damaged by a nearby lightning strike.  Temporarily I have replaced the remote tuner with a 10 foot piece of RG-6A/U coax (I had some handy) running into my basement shack to a manual tuner.  I used an online transmission line loss calculator to calculate the worst case loss in the 10 foot run of coax and, on 20 meters, it is under 0.6 dB.  Does anyone see any problems running like this permanently instead of buying a replacement remote autotuner?  Am I missing something obvious? Output typically is 100W or under.

Gene
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N4UM
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« Reply #1 on: September 23, 2012, 10:11:51 AM »

I should work fine.  You might want to consider replacing the RG-6 with something along the lines of high quality RG-8.  I run 13 feet of Davis RF Buryflex from an autotuner on my back porch to a 23 foot flagpole in my yard and it works reasonable well. 
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WB6BYU
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« Reply #2 on: September 23, 2012, 04:44:45 PM »

RG-6 probably has lower loss than RG-8 in that application, depending on your
load impedance.

(For a 2000 ohm load, RG-6 has 0.5dB loss and RG-8 has 1dB loss.)

One other important factor to consider is the impedance that your tuner sees,
especially on 80m.  The length of coax affects that, as well as the antenna
impedance.  Lower impedances (especially below 5 or 10 ohms) usually mean
higher losses in the tuner, which need to be considered in addition to the
coax losses.

Power handling capability will also depend on the antenna impedance and
the power level.


But as long as you are willing to accept the losses (which may not be very
much in this case if the antenna impedance is reasonable) then there is no
problem with such an installation.
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KE5JPP
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« Reply #3 on: September 24, 2012, 03:40:31 AM »

Thanks, guys, for the responses.  I stuck with RG-6 since I have some high quality RG-6 on hand and, for the load impedances that the antenna supposedly presents on my bands of interest, the transmission line loss calculator showed lower loss for the RG-6 over the RG-8.

Gene
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W8JI
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« Reply #4 on: September 24, 2012, 05:01:46 AM »

Thanks, guys, for the responses.  I stuck with RG-6 since I have some high quality RG-6 on hand and, for the load impedances that the antenna supposedly presents on my bands of interest, the transmission line loss calculator showed lower loss for the RG-6 over the RG-8.

Gene


Be careful of transmission line loss calculators.

If a calculator ONLY asks for SWR, and does not have an input for complex load impedance, the calculator will be wrong for short transmission line losses. Short transmission lines have loss that is dominated by actual line current, and not line SWR.
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KE5JPP
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« Reply #5 on: September 24, 2012, 07:05:26 AM »

Thanks, guys, for the responses.  I stuck with RG-6 since I have some high quality RG-6 on hand and, for the load impedances that the antenna supposedly presents on my bands of interest, the transmission line loss calculator showed lower loss for the RG-6 over the RG-8.

Gene


Be careful of transmission line loss calculators.

If a calculator ONLY asks for SWR, and does not have an input for complex load impedance, the calculator will be wrong for short transmission line losses. Short transmission lines have loss that is dominated by actual line current, and not line SWR.

I used http://vk1od.net/calc/tl/tllc.php.  The complex load impedance estimates came from EZNEC for the particular antenna I am using.

Gene
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M6GOM
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« Reply #6 on: September 24, 2012, 02:21:50 PM »

So is the feed from the ATU in the shack to the base of the inverted L just 10ft long or is this RG6 tacked onto the end of the co-ax that the SGC230 was connected to?
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KE5JPP
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« Reply #7 on: September 24, 2012, 03:06:17 PM »

So is the feed from the ATU in the shack to the base of the inverted L just 10ft long or is this RG6 tacked onto the end of the co-ax that the SGC230 was connected to?

No, it is a total of 10 feet of RG-6 coax only to the tuner.

Gene
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W5WSS
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Posts: 1732




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« Reply #8 on: September 24, 2012, 06:20:35 PM »

The additional loss is due to mismatch.  10ft from the output of the tuner to the antenna places the line in the mismatched zone.

Vk1od line calculator takes these factors into account.

.6 of 1 dbd of worst case loss is acceptable IMHO.

ahem another lost auto tuner....

If one is going to place an expensive sensitive electronic network out there in the lightning vulnerability zone then please for everyone sake use a wander lead out ahead of the tuner and connect the center conductor at the antenna so-239 to a ground rod the tuner should be in a metallic box with the tuner more protected inside. The metal box may be grounded not the tuner but the metal wx proof box. Saying Again Attach the wander lead as close to the antenna feedpoint ahead of the network for bypass to the ground rod diversion. The pulse of a stroke can still damage the sensitive equipment anyway but this technique helps.

PS I am not suggesting that you lost your tuner for not doing these suggestions but just taking the opportunity to reiterate
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