-
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.
Need More R
- Thread starter C2
- Start date
while lowering the radials will increase the amount of R in Z, one of the easiest ways to increase R is to use capacitive top hat loading. as for the input matching this can be had by utilizing a *shunt feed type matching network. you must also make sure that the radiating element is cut to 5/8 precisely where it attaches to or "taps" the inductor. this* also places the entire antenna system at direct DC ground potential.
You are saying the test setup is defective because I observe higher receiver strength on a dipole over a home brew 5/8 wave antenna, and that is based on the given of the 5/8 wave antenna having a good match? I also noted a significant decrease in receive of various repeaters in the area when comparing the dipole to the home brew 5/8 wave antenna.
One thing that I had considered is that the PVC used to construct the 5/8 wave antenna was influencing its performance, but now I'm not sure after rebuilding it as a base loaded 3/4 wave antenna, which performs better than the dipole. There must be something about the antenna that leads to its poor receive performance. The quality of the match would not give an indication of its performance as an antenna. Case in point, I could replace the antenna with a perfectly matched dummy load.
I do recall seing cap hats on other 5/8 wave designs...
I'll rebuild the classic design again and start over.
Ground elements will be 38.4 end to end and the radiating element will be 48 inches long (The ARRL text says to make it 47 inches long, however).
The coil will be 10.5 turns around 3/4 inch PVC (24.75 inches of wire) and tapped about 4 turns from the top (The ARRL text shows the tap at 3.75 turns from the top).
|
|
(G)-//////////-------------<|
| (+)----'
|
I don't know what a dummy load has to do with anything here. A perfect dummy load exhibits infinite radiation resistance. A perfect antenna/matching system exhibits radiation resistance equal to the source impedance feeding it.
Given a good match means whatever antenna under test is working to it's capability. If you see two 5/8ths physical length antenna both with low SWR showing 6 dB difference something is way off with the test setup. 6dB is quite a difference.
Yes, you are missing something with the test. Maybe try more distance and hook up several of those mighty dipoles to prove your method.
Seasoned VHF operators will all tell you reception can vary markedly by simply moving a few feet.
Try a 100 watt carrier for 5 minutes straight into any antenna suspected of dielectric losses in it's construction. Sniff, record results.
HiDef,
'Infinite' radiation resistance, in a dummy load? No. A dummy load exhibits a constant 'radiation resistance' at all (almost) frequencies, and it isn't infinite by any means. Remember that 'radiation resistance' is like the 'resistance' in impedance.
"Given a good match means whatever antenna under test is working to it's capability." I have to disagree with that, it depends on how you are meaning "Good Match", and how you do the measuring of that 'match'. If using an SWR meter, the statement is NOT true in all cases. There are many 'R +/- J' that will appear to be 'R50 +/- J0', and be far from it. ('J' = 'X' in the case of MFJ analyzers.)
Moving an antenna can change it's receiving/transmitting characteristics. Yes, it certainly can. But these test figures are all relative, not absolute. Meaning that considering that all of these tests are done in the same circumstances/conditions, they are 'relatively' true comparisons. Is you were to move one of those test antenna positions relative to others in the same test, those readings are no longer 'relatively' true. If the testing conditions are valid to start with, and if all tests are done in the same conditions, then those results are true for those conditions and comparisons. About like testing the hearablity of a particular sound, and then changing the strength of the signal fed to one speaker and not the other speaker being tested. (That's not the greatest analogy in the world, is it? Oh well.)
I do agree that those 3 and 6 dB results are not exactly believable. Can't say why, but they certainly are not what's normally seen, in a standardized testing situation.
- 'Doc
'Infinite' radiation resistance, in a dummy load? No. A dummy load exhibits a constant 'radiation resistance' at all (almost) frequencies, and it isn't infinite by any means. Remember that 'radiation resistance' is like the 'resistance' in impedance.
"Given a good match means whatever antenna under test is working to it's capability." I have to disagree with that, it depends on how you are meaning "Good Match", and how you do the measuring of that 'match'. If using an SWR meter, the statement is NOT true in all cases. There are many 'R +/- J' that will appear to be 'R50 +/- J0', and be far from it. ('J' = 'X' in the case of MFJ analyzers.)
Moving an antenna can change it's receiving/transmitting characteristics. Yes, it certainly can. But these test figures are all relative, not absolute. Meaning that considering that all of these tests are done in the same circumstances/conditions, they are 'relatively' true comparisons. Is you were to move one of those test antenna positions relative to others in the same test, those readings are no longer 'relatively' true. If the testing conditions are valid to start with, and if all tests are done in the same conditions, then those results are true for those conditions and comparisons. About like testing the hearablity of a particular sound, and then changing the strength of the signal fed to one speaker and not the other speaker being tested. (That's not the greatest analogy in the world, is it? Oh well.)
I do agree that those 3 and 6 dB results are not exactly believable. Can't say why, but they certainly are not what's normally seen, in a standardized testing situation.
- 'Doc
SRI Doc, you have it backwards.
Radiation resistance - Wikipedia, the free encyclopedia
Radiation resistance is that part of the impedance which contributes to RADIATION of a signal. A perfect dummy load radiates nothing therefore it has infinite radiation resistance. Normally it will have a 50 ohm resistance which is not to be confused with that portion of the impedance useful for transmission. Real world dummy loads do not exhibit infinite radiation resistance but the better ones exhibit numbers in the thousands of ohms.
Okay, just a different way of 'looking' at it that I don't subscribe too. It's also a very theoretical concept, and if you view the whole thing in that way (radio/radiation/etc.) if probably holds true. I've also found that thinking of it in a more 'real world' type way (old fashion) certainly makes it easier for me to understand and keep track of. Sort of like 'electrons' and 'hole theory'. Traveling electrons make sense to me. Those traveling 'holes' are a different story! 
If you understand both ways of thinking about it, you can get away with mixing the two. But if you don't explain what you are doing in that respect, oh are you gonna have fun!
- 'Doc
If you understand both ways of thinking about it, you can get away with mixing the two. But if you don't explain what you are doing in that respect, oh are you gonna have fun!- 'Doc
In the context of this thread we have two 5/8 wl antenna. One is fed with a series coil, the other with a series coil tapped from the cold end for impedance match. Both were said to be matched. Same antenna design. The feed is the variable. The experiment is said to yield 6dB more signal strength on an SA on one antenna. No mention is made of anything other than flat line feed. This leaves no room for anything to "appear" differently than it is at the load end. I'm assuming C2 has an accurate way to check the match. Maybe that IS the problem?
As Doc allured to, the antenna designs are not the same. A series coil in the base is not the same design, IMO.
I suspect that Doc is correct, that the second antenna is probably a base loaded, shortened antenna of another wavelength, probably a base load 3/4 wave antenna. Also note that the loading coil is wound tight as to fit inside the PVC rather than outside as on the first.
More believable to me is that the first antenna has a very lossy impedance match or some other problem...
The test setup is too basic to present any problems and I also tried the antenna up on the roof the same place where I had my dipole - and various repeaters showed a significant reduction in signal strength (3 bars vs 5) and some repeaters that were full quieting were then full of noise.
At any rate, I'll repeat the experiment after rebuilding a new 5/8 wave antenna.
That is interesting, bob85, and I also found one-level-up quite useful as well.
I had considered copying the Maco V58 desing, with a single turn tapped coil...
Mobile antennas, short verticals, loading coil loss,and loading coil current
I had considered copying the Maco V58 desing, with a single turn tapped coil...
Mobile antennas, short verticals, loading coil loss,and loading coil current
I think Doc may have alluded. I haven't seen him allure nor would I be interested.
Your thumbnails show 2 antenna which differ only in feed design. If there is more going on than that your drawing is incorrect and incomplete. How can someone discuss something that isn't represented correctly?
I suggest you check out the many fine articles on antenna test ranges. Most of the better ones are IEEE publications. Antenna testing isn't as simple as it first appears.
As for repeaters changing strength: path loss is in constant change this time of year. Check out the APRS tropo maps. Valid experiments seek one variable at a time.
Have you applied some of that test equipment to see just what a delta of 3 to 5 bars really means?
- No one is chatting at the moment.