Sidebar Sidebar
  • You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.
  • Click here to find out how to win free radios from Retevis!

100 feet of RG-58A/U SWR effects shown visually.

hookedon6 said:
clearly your reading skills are sub par.

look at the FIRST post where he says QUOTE:"This shows the effects of coax on SWR "
Click to expand...

Interesting to note you missed the line before that:

The DB said:
Below are many pictures showing what 100' of RG58A/U coax does to your apparent SWR reading.
Click to expand...

This exercise was meant to show what losses in a feedline can do to an SWR reading with the load at the far end of such a coax from the meter. Bob85 is on target with what I said and meant here.

hookedon6 said:
and then he said QUOTE:" Less length and the effects will be lessened, more length and the effects will be greater."

... and in post #2 , I said that was wrong

you seem to like to quote a lot, have you ever really read the references?

Originally Posted by Reflections, by M. Walter Maxwell
21)...22) SWR in a feed line cannot be adjusted or controlled in any practical manner by varying the line length (ref. 7, p. 51).

sooooooo, WHY do you keep insisting that coax lenght affects VSWR???


I'm done with you,..... conversations with pieces of furniture are seldom productive
Click to expand...

But what I said is correct. Read #21 that both bob85 and I quoted in this very thread, and you referred to above. I'll quote it again for you.

Reflections said:
21) SWR indicators need not be placed at the feed-line/antenna junction to obtain a more accurate measurement. Within its own accuracy limits, the indicator reads the SWR wherever it is located in the line. The SWR at any other point on the line may be determined by a simple calculation involving only the SWR at the point of measurement, the line attenuation per unit length (available in a later installment), and the distance from the measured point to the point where the SWR is desired.
Click to expand...

The formula mentioned in this quote includes the attenuation effects of the feedline. This attenuation effect affects signals in the feedline in such a way that the longer the feedline is the more attenuation of that signal is present. The further down the feedline the meter is from the load the lower the SWR reading will be on the meter. Thus the longer the feedline the more of an effect will be shown (namely a lower SWR reading), and the shorter the feedline the less of an effect will be shown (a reading closer to the actual SWR from the load).

You can take a load with an infinate impedance, and, given enough feedline length, make it appear to be a perfect SWR match on an SWR meter. This does not mean you have a perfect SWR match at the antenna, it is just what is shown on the meter beyond a length (a pretty long length mind you) of feedline.


The DB
 
The DB said:
You can take a load with an infinate impedance, and, given enough feedline length, make it appear to be a perfect SWR match on an SWR meter. This does not mean you have a perfect SWR match at the antenna, it is just what is shown on the meter beyond a length (a pretty long length mind you) of feedline.


The DB
Click to expand...


This I can attest too. While installing a point-to-point UHF transmitter on 959.8275 MHz I needed a 6 dB RF attenuator and needed to get the TX on the air before the supplier could deliver it. I decided to use cheap RG-58 as an attenuator until the Bird attenuator arrived. While testing out how much cable I needed for 6 dB loss I decided to see what the SWR would be if the end was open. With about 10 watts into a full roll of unterminated RG-58 coax I saw a nearly perfect SWR at the TX. on 959 MHz. The longer the cable the greater the effect at the opposite end. In this case it was transforming (for lack of a better term) the infinite impedance of an open circuit into nearly 50 ohms. As the cable was shortened that effect became less and the impedance see by the TX rose up from that 50 ohms figure thereby showing less cable has less of an effect than the longer cable.


Now, lets leave the personal opinions about one's intelligence level or ability to read and get on with the discussion.
 
And from what I've been able to gather, that result was a 'result' of the resistive losses of that RG-58 being the same or greater than it's impedance at the frequency it was used on. At least that's the commonly accepted reason.
- 'Doc
 
W5LZ said:
And from what I've been able to gather, that result was a 'result' of the resistive losses of that RG-58 being the same or greater than it's impedance at the frequency it was used on. At least that's the commonly accepted reason.
- 'Doc
Click to expand...


But the resistive losses remain constant regardless of the frequency while the actual losses increase with increasing frequency. RG-58 has a loss of about 21.5 dB per 100 feet at 1 GHz. This equates to an SWR of [FONT=Arial, Helvetica]1.18:1 for 100 feet of unterminated RG-58.[/FONT]
 
The common way of determining impedance is by measuring voltages. That means that at some point the resistance of that unterminated cable will be 50 ohms, right? Or maybe the impedance of that unterminated cable will be 50 ohms? If so, that means that at that particular length the SWR will -seem- to be 1:1, within the range of reasonable measuring accuracy. So since there's no load on that cable and the transmitter will 'see' a desirable load, it's going to act as if it's a terminated load of the desired value and behave accordingly. The transmitter will be having a ball, and there's not really going to be any indication from that power/SWR meter that things aren't 'right'. Right? It also means that if there's any radiated signal at all (doubtful) it isn't going to amount to squat.
No, none of this is 'scientifically' explained at all, but it IS how it turns out. About all that can really be said for all of that is that SWR is a terrible way of determining how well an antenna is working. It doesn't tell you enough to make a knowledgeable -guess- as to what's happening or why.
So what's my point? Simple, find out what you are talking about before talking about it or you only make yourself appear as uninformed as you are. No, I'm not 'dissing' anyone. Or maybe I am, but I'm including me in that too.
- 'Doc
 
In the grand scheme of things, Standing Wave Ratio is nothing more than an indicator. A certain type of antenna should have an SWR of say A. Put this coax in between the antenna and meter it should now show an SWR of B. If either of those are to high or to low there is an issue after the meter that should be taken care of.

Unfortunately, most people who put up an antenna don't know this. They simply follow instructions that came with the antenna which only includes SWR. The manufacturer of an antenna is not going to tell the people they sell their $15 antenna to that they need a $300+ antenna analyzer to tune it properly. This leads people who are new to the field (and others as well) to think having a low SWR is the most important part (and quite frankly the only part) of properly tuning an antenna. Luckily for them with the antennas they tend to purchase a low SWR will provide useable results.

Unfortunately the learning curve to go from this point to knowing what is actually happening is quite large. Going from watching one variable to four or five and grasping what they all mean and how they relate to and interact with each other is beyond most people for one or two antennas. Add in the cost of the equipment that can actually tell you what those variables are and your completely beyond what they are willing to invest in time and money, and that is if they know that there is information there to be learned and equipment to purchase.

How many people do you know would purchase a $300+ piece of equipment and spend hours of their free/family time studying just to tune their $15 antenna to be as good as it can be?


The DB
 
When measuring SWR on an antenna having a small amount of reflected power, the length of the feedline between the bridge and the antenna may affect your SWR reading. An example of this is a 70-ohm antenna being fed with 50-ohm coax. Different lengths of feed-line will give you small differences in SWR readings because at certain lengths, the mismatched feed-line starts to act like a series matching section. In the case of a 70-ohm antenna fed with 50-ohm coax, if the feed-line is a half wave long, the SWR will measure 1.4:1. At some particular length of feed-line and on one frequency, the SWR will measure 1:1 because that length of that feed-line transforms the impedance to make a match. Some hams have adjusted their feed line length to get a perfect match. This is called "tuning your antenna by tuning your feed-line." With other feed-line lengths, you will measure something different. Suppose the impedance of the feedline and the antenna are perfectly matched. Then there is no reflected power. You will get a 1:1 reading on the SWR-bridge with any length of feed-line
 

dxChat
Help Users
  • No one is chatting at the moment.
  • @ kopcicle:
    If you know you know. Anyone have Sam's current #? He hasn't been on since Oct 1st. Someone let him know I'm looking.
  • @ BJ radionut:
  • dxBot:
    535A has left the room.
  • @ AmericanEagle575:
    Just wanted to say Good Morning to all my Fellow WDX members out there!!!!!
  • @ BJ radionut:


      Top Bottom
      Back