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COAX QUESTION




 
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W2PFY
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« on: March 16, 2017, 03:10:03 PM »

I have about 300 feet of RG-213  (50 ohm) and about 100 feet of RG-11 (72 ohm)cable. I have in the past had this coax in use where the last 100 feet is the RG-11. It worked but I don't know if I was losing power by this mismatch in coax?

So my question is just how much power am I losing by doing this and what is the overall impedance at the end of this line.

Some background: I had to take the system down because of a logging operation on the property and one neighbor was telling me but not complaining that they could hear me on their AM radios on other antennas that were in the near field. So that is why I want the 75 meter antenna as far away as is possible using all the coax I have on hand. I am a neophyte when it comes to some aspects of RF transmission so let me know what I don't know please Huh Huh   
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« Reply #1 on: March 16, 2017, 03:37:11 PM »

If the antenna is matched at 50 ohms and you feed it with 75 ohm coass, then you have a 1.5 swr.  Look at the coax loss chart for your rg 11 and calculate the loss.

Then, look at a smith chart, see what the input impedance is at the feed point of the 75 ohm coax and plug that into the loss calculation for the 213.

Or, match the antenna to 75 ohms  and have a 75 ohm input at the connection of the 213.  Now calculate the loss on your 213.

If you choose to just string it all together, you may find your swr at the shack end is quite low due to all the loss in the coax and mismatch increasing said loss?

--Shane
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Or, use series transformers to match the antenna to the 75 ohm coax and the 50/75 ohm Junction. You'll have loss due to the swr in the series sections, but I'll wager its less than the loss due to the mismatch on 400 or more feet of line.
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WD8BIL
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« Reply #2 on: March 16, 2017, 03:58:13 PM »

typical loss at a 1.5:1 is 4%. At 2:1 it's like..11%

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« Reply #3 on: March 16, 2017, 04:16:09 PM »

Hi Terry,

On 75M and below, the loss is insignificant.

There is an advantage... leave some extra RG/11U at the end to play with. By adding and subtracting 5' or 10' at a time, you may be able to get the input match near 50 ohms in the shack. It all depends on the actual the dipole's feed point and the impedance where the RG/11U connects to the RG-213.  (the swr will always be 1.5:1 or whatever along the entire line)

I do that here on several antennas to dial in the match to my favorite operating freq. This way same band antennas are close to the same impedance and makes fast A/B switching quicker without having to retune.

It may also make solid state rigs happier.     Pi-network tube rigs will not care.

T
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« Reply #4 on: March 16, 2017, 06:09:01 PM »

1/4 wavelength of 75 ohm coax connected to the antenna and remaining length to the xmtr being 50 ohm will cause a step up in impedance at the antenna to about 120 ohms.  Your length being a 100 ft is close to a 1/2 wavelength at 3.88mhz with a usual VOP of the coax being 83%. So, your impedance at the antenna would be back down to about 50 ohms.  Not sure exactly how much loss you would have using the 75 coax.  The 75 ohm coax by itself does not create a mismatch, it's the load at the end of the coax.  There would be some insertion loss due the connectors which would occur regardless of the type coax.
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Pete, WA2CWA
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« Reply #5 on: March 16, 2017, 08:06:28 PM »

There's several online calculators for determining coax loss at a particular length, frequency, etc.
Here's one: http://kv5r.com/ham-radio/coax-loss-calculator/
Obviously coax age and quality are additional factors that can undermine coax loss calculations.
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« Reply #6 on: March 18, 2017, 12:21:25 PM »

The answer is ........ There is no way to guess what the impedance is going to be at the end of one / two / three pieces of dissimilar coax lines of unmeasured lengths.  And these measurements are frequency dependent.

I think of quarter wave coaxes as the pivot of a playground see-saw.

At a  SPECIFIC frequency, a 100 ohm load/antenna ( the high end of the see-saw ) will be transformed to 50 ohms ( the low end of the see-saw ) by a 75 ohm 1/4 wave line. ( the pivot point )

This analogy works for any impedance of coax or open line.

Moving to a 1/2 wave length is like putting two see-saws end-to-end.

Using the above example, a 100 ohm load will transformed to 50 ohm in the first 1/4 wave and then be transformed back to 100 ohms by the second 75 ohm coax see-saw.

So regardless of the coax impedance, a 1/2 wave length will repeat the load impedance .... 100 ohms to 100 ohms   25 ohms to 25 ohms..... and so on.

A quarter wave length will flip the load impedance around the coax impedance.

If you have a VNA like a AIM 4170 , MFJ copy, or even a simple swr meter, you can take advantage of these line impedances to create a "matched"
( not necessarily low loss )  50 ohm input that your transmitter will like.

But you have to measure the load at the end of your coaxs before you can know just what lengths will work for your particular antenna ( load ).

Using 75ohm and 50ohm lines in a system can be another tuning tool.

Knowing which impedance and length goes at which end of the feedline is made much easier by using a VNA.

Don W4DNR




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« Reply #7 on: March 20, 2017, 03:19:36 PM »

Thanks for the education gentlemen! A person can never know it all but if an individual has a question about anything related to transmitters or receivers,they need not look any further than here an AMFONE!

Thanks for the link Pete, I am stuck on that source going on for a week now Grin Grin

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