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Author Topic: Impedance bump through antenna relay  (Read 4626 times)
KK7UV
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« on: October 04, 2010, 03:20:55 AM »

I am using an Ameritron RCS-8V Remote Coax Switch.
I measure the impedance (with MFJ antenna analyzer) at the end of a feedline for an antenna as 148 +j8.
I then attach that same coax to one of the RCS-8V "antenna" connectors, and then measure the impedance at the common coax connector of the RCS-8V as 1577 + j111.

The only difference in the measurement lash-up is the RCS-8V relay circuit and a 1-foot piece of coax from the antenna analyzer to the RCS-8V.

Why such a large bump?
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W5DXP
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« Reply #1 on: October 04, 2010, 04:39:21 AM »

Why such a large bump?

What frequency?
--
73, Cecil, w5dxp.com
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KK7UV
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« Reply #2 on: October 04, 2010, 08:25:29 AM »

Correction - - I meant to type 577 +j111.  Still a large bump though.

Frequency is 3.7Mhz.  Antenna is a 80m dipole at 75 feet fed with 80 feet of new RG-8X.
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N3OX
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« Reply #3 on: October 04, 2010, 10:07:25 AM »

That's not a bump, that's a bad connection or something.

Try cycling the relay a few times, make sure it's engaging.

Sometimes those coax switches develop a problem where they oxidize enough to block tiny signals while still passing large
transmitted ones... if you haven't transmitted through it in a while, it might not be making good contact.
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
KK7UV
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« Reply #4 on: October 05, 2010, 04:50:20 PM »

I took the relay contacts apart and cleaned them.  Made sure the relays are making good mechanical contact and tried again.  No change. 

I get a good SWR dip at around 3.7 MHz with the antenna analyzer connected directly to the antenna feedline.   SWR around 3:1 at the lower band edge and 4:1 at upper band edge.

I connect the antenna feedline to one of the RCS8V antenna ports (I've tried them all with the same result), then I connect the analyzer to the common switch port with a 1-foot length of coax, and the analyzer shows a strong SWR dip at 3.2 MHz and quite high SWR's across 80m band.  The short piece of coax is good, and I've tried two other pieces as well.  All read the same.

I'm stumped.
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WB6BYU
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« Reply #5 on: October 05, 2010, 07:11:48 PM »

Is the switch casing grounded?  Are you using a balun?

A quick check would be to connect the jumper coax to the antenna coax with a barrel - you should still
get about the same reading.  Then touch the barrel to the switch case (finding a good spot of bare metal)
and see if the impedance changes.  If it does, the likely cause is common mode currents - they will
change depending on the coax length and the path to ground at the rig end.

The other test would be to connect a dummy load through the relay to see if it still looks like 50 ohms.
This doesn't require a high power load - a resistor soldered into a PL-259 (as are often sold for CB use)
is fine.  If you still see 50 ohms through the switch, it should be fine.
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AA4PB
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« Reply #6 on: October 06, 2010, 05:00:19 AM »

On 80M, the physical length of the relay contacts is much too short (in terms of wavelength) to cause an impedance bump like that. You must have a bad connection somewhere.
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KK7UV
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« Reply #7 on: October 06, 2010, 05:00:02 PM »

Okay - now we're getting somewhere.

The switch case is grounded to a rod 3 feet away, which is also bonded back to the shack/house system.

I am using a homemade 4:1 current balun similar to the Balun Designs design.

I connected a jumper coax to the antenna coax with a barrel and touched the barrel to the switch case and the impedance changed.  So then, I have a common mode current issue apparently.  Perhaps I should check my balun again.

I tried the other test with the dummy load through the relay.  It is 50 ohms.
So the switch should be fine, and I need to look at my balun/antenna/feedline.



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WB6BYU
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« Reply #8 on: October 06, 2010, 08:54:44 PM »

Quote from: KK7UV
Okay - now we're getting somewhere.

Good!  Common mode currents can cause some very quirky symptoms.


Quote
I am using a homemade 4:1 current balun similar to the Balun Designs design.


Why the 4 : 1 balun?  Is the dipole fed off center?
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W8JI
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« Reply #9 on: October 11, 2010, 01:29:43 PM »

Okay - now we're getting somewhere.

The switch case is grounded to a rod 3 feet away, which is also bonded back to the shack/house system.

I am using a homemade 4:1 current balun similar to the Balun Designs design.

I connected a jumper coax to the antenna coax with a barrel and touched the barrel to the switch case and the impedance changed.  So then, I have a common mode current issue apparently.  Perhaps I should check my balun again.

I tried the other test with the dummy load through the relay.  It is 50 ohms.
So the switch should be fine, and I need to look at my balun/antenna/feedline.





I've got bad news for you. The balun Designs 4:1 current balun, if it is a single core balun, is not a working balun at all.
It is a terribly unbalanced voltage source.

So if you copied their single core 4:1 balun, you don't have a balun at all. It is not unbalance, it is not balanced, it is in a land between the two.

I hope you didn't hurt the RCS8V!!! NEVER clean contacts for problems like that, especially when they are gold flashed.

If you want to read why that balun design cannot possibly work as a balun, read this link:

http://www.w8ji.com/balun_single_core_41_analysis.htm

73 Tom
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KK7UV
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« Reply #10 on: October 13, 2010, 03:55:47 AM »

It is a dual core 4:1 balun feeding a 20% OCF Dipole.  It is similar to BalunDesign's model 4115ocf.

It bench tested good with a 200-ohm resistor.  I'll be taking the antenna down to check connections and wire lengths.

I simply rubbed the contacts with a cloth and spit. No harm done?

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W8JI
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« Reply #11 on: October 15, 2010, 01:50:02 PM »

It is a dual core 4:1 balun feeding a 20% OCF Dipole.  It is similar to BalunDesign's model 4115ocf.

It bench tested good with a 200-ohm resistor.  I'll be taking the antenna down to check connections and wire lengths.

I simply rubbed the contacts with a cloth and spit. No harm done?



A floating resistor test tells you nothing about the balance performance of the balun, or even if it acts like a balun at all.

Perhaps the balun does not have much common mode impedance, or the CM impedance is a high-Q reactance and has inadequate choking for the CM impedance of your transmission line??

73 Tom

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KK7UV
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« Reply #12 on: October 16, 2010, 04:46:25 AM »

Thanks Tom.  I read through your updated material on the "4:1 balun design and operation" subject.

Today the antenna is coming down to check for good connections at the balun and confirm wire lengths on the 20%/80% fed OCF dipole design.   As I understand, I should then experiment with different feed line lengths.  If that doesn't work, I guess it's back to the bench with the balun.

Steve

UPDATE***
I took the dipole down and measured the elements again.  27.4 and 109.6 feet - I'm good there.

Balun on the bench - - I am only able to check the balance with a 259B and it looks good there.  Checked all balun connections.

Hoisted dipole back up to 75-80 feet and nothing has changed SWR/impedance-wise.

Guess it's time to rewind to the balun and get someone with the proper test equipment to get the correct values.



 
« Last Edit: October 16, 2010, 02:21:29 PM by steve flood » Logged
W8JI
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« Reply #13 on: October 17, 2010, 05:52:36 AM »

With an OCF antenna, you might need more than one common mode choke.

Also, try turning the balun terminals around on the antenna side.
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W0BTU
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« Reply #14 on: October 17, 2010, 08:52:54 PM »

I'm not knocking baluns, ununs, and chokes; they have their places. However, a lot of amateurs have built efficient antennas and made contacts way before ferrite and iron powder toroids were ever thought of.

Whatever happened to such things as the balanced tuner?

http://www.w0btu.com/files/antenna/W0BTUBalancedTuner.jpg

Those coils (and I have several other plug-in coils for all HF bands) don't get hot even at the legal limit in a contest.
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