PDA

View Full Version : Learned Lesson About Antennas - Just Not Sure What



KJ6CNS
10-19-2009, 08:42 AM
:confused:

Hoping you fine folks could tell me what I learned today ha! Very New ham. Have a kenwood F6A HT. Getting 857D in the next couple of weeks. Mounting a Diamond X-30 on the roof tomorrow.

So Tonight I did a test at the dinner table of the antennas Ive gotten in the last two weeks. RX 70cm

1 - Random Wire I cut.
2 - Stock Rubber Duck
3 - Diamond SRH320 (Popular aftermarket HT antenna)
4 - Diamond X30 vertical roof(base) antenna using 3 foot pigtail SMA - UHF bought from HRO

The difference between the 4 is so slight Im amazed.

Why is that?

A) Antenna location(roof/outside vs indoors at a table) is infinitely more important than antenna design.
B) The receiver in an HT is "garbage" not "poor compared to mobile or base unit" but "garbage"
C) My testing method is flawed
D) Something I don't know thus haven't listed.

I know A & B both add to the issue but one has to be more important.

W0UZR
10-19-2009, 10:20 AM
If you are testing all those antennas to see how well they work on a local repeater, then they will all work very well, and you probably won't notice a difference.

How about trying the roof antenna and compare the others with a repeater that is a quite a bit ferther away or simplex and see what you get for a difference?

~~EDIT~~

I forgot you said you are testing 70 cm. I never been on 70 cm so I don't know if there are repeaters there or not.

AB8XE
10-19-2009, 12:44 PM
A) Not infinitely more important, but very important
B) I don't have any reason to knock that receiver.
C) Somewhat. Even a dummy load will receive a strong signal. Test receiving on weak signals.
E) Finding that out is half the fun of ham radio. :) Try researching into receive only antennas, they have horrible gain but directivity to allow you to lower the noise floor, thus hearing weak signals better. You need to ask yourself what you want your antenna to do for you, and test accordingly.

W4HAY
10-19-2009, 01:03 PM
To quote my professor in Electomagnetic Waves and Fields 301:
"Antenna design is 15% knowledge, 5% luck, and 80% witchcraft!"

The receiver's AGC is probably compensating for the antenna differences. As others have said, weak signals will tell the story.

G4ALA
10-19-2009, 02:27 PM
Hi,

If a resistanceless short antenna is matched (that is, the reactance is cancelled and it is driven into the correct load) the power received from a particular field strength is independent of the length!

This probaly says something significant about the structure of space-time. No matter.

The radiation resistance falls as the short antenna gets shorter, and the matched resistive load will therefore be less and less. Real world resistive losses will overwhelm this interesting effect.

Large diameter structures do better than skinny ones. Of course, at VHF the antennas are pretty stubby compared with those for HF signals. This means that the different antennas might not differ an much as we might expect.

Another thing to consider is the truly enormous sensitivity range a modern transceiver can hear. S9 is typically 50 microvolts into 50 Ohms matched(it is at HF, anyway) The power difference between S9 and S1 is 48 db or around 63 thousand times. The range of most receivers extends up at least another 40 db, making the whole range around 700,000,000 to one. That is part of the reason why a QRP station, on the other side of the world, can still be heard although it is running less power that a human voice.

What I am trying to say is that any "small" differences (could be even 20 db) would probably go largely unnoticed.

My advice is use the largest antenna you can. Prefereably, if you can, place it outside.

73 and good luck

John
G4ALA

N0SYA
10-19-2009, 02:40 PM
Remember:
Itty bitty antennas (such as the stub ducks) will only transfer a smidgen of what rf power huge behemoth antennas (like a full-wave loop or log periodic for 160m) will, no matter the losses from impedance or design frequency mismatch. And it's kinda impossible to recover more power than the transmitter is actually emitting, if they output 1w to a 90dBi dish pointed at your 90dBi dish all you can get is 1w and that is under perfect conditions that do not exist in reality. That and antennas apparently re-radiate some of the rf power they receive, so you only get a portion of the total available signal.

But always remember:
"Rf gatta go somewhere!"

K0RGR
10-19-2009, 03:59 PM
If you are really located where your profile says you are, the repeaters are mostly on top of mountains, and you could work the close ones with a wet noodle. The real test is to see how well you can work stations in, say, Long Beach on 446.0 simplex. The difference in antennas will become readily apparent. If the station you're working is 'line of sight', there isn't much difference. It's when the stations are on the fringe that the antennas matter.

The practical differences become more apparent when you live in flat country. I have a hard time working the local UHF repeater from my house with an HT, but with a good outside antenna, I can work one about 60 miles away. BIG difference...

KM3F
10-19-2009, 05:41 PM
If your QTH is within an easy low loss path to the repeater you are testing with, remember the repeater reciever is apt to be quite high and will sense very low signal levels from any source on the input frequency of the pair.
This means any antenna you try will radiate enough for the repeater reciever to hear.
The sgnal back from the repeater transmitter is apt to be about 100 watts +/- effective radiated power (ERP) and any antenna you have connected will recieve that signal in an often (overwhelming manner) called FULL QUIETING.
You can't really test very well, antennas within a close distance to a repeater but should go to a fringe areas where the path both ways has enough path loss to show up when any peticular antenna has greater/lesser performance over another.
Another way might be to use a 'switchable attenuator' between the antenna and radio to simulate fringe signals and test the same way.
The more attenuation you can switch in and still hear with, shows the best antennas 'situation'.
Only possible difference is if the case of the HT is not RF tight and lets recieve signal get into the radio by other means, bypassing the attenuator to some degree..
An example not the same as what you are doing, is a repeater with 80 watts to the antenna on a 2700' high mountain is usefull to me when out mobile at distances of more than 70 miles.
The RF path back often needs to be found while mobile because it's not solid due to changes terain while mobile.
In one case I used a dual band antenna vs a 5/8 wave single band type. The difference is night and day between the two antennas, showing up the gain factor between the two types.
Also the power output of the radio has a great effect, with the recieve back staying the same.
Most of the time you can hear a repeater at great distances but not be able to get into it due to your lower power, lesser antenna gain and physical location.
A way to get a feel for this is to note the lowest number of bars of signal strenght you can "get back" to the repeater, at.
Hope this gives you a better feel for overall effects on operation.

KB3X
10-19-2009, 07:18 PM
:confused:
C) My testing method is flawed



You failed to rely on anecdotal evidence. It's the most popular form of antenna evaluation.

KB2FCV
10-19-2009, 07:39 PM
If you are very close to the repeater, it won't matter what antenna you have, you'll most likely hit it with no problem. I have a repeater less than a mile from me that I keyed up by accident with no antenna and it still came up 1/2 scale. If properly installed with good coax, etc, the outdoor antenna will outperform your other choices.

KJ6CNS
10-19-2009, 07:46 PM
Thank you to everyone for replying! I think I did a poor job of explaining the results and my expectations.

I was looking for additional quieting. I thought that by the time I got to the biggest antenna, I would have full quieting. However, the improvement was very minor. So much so, its difficult to believe I could work a weak signal with one and not the other.

AG3Y
10-19-2009, 07:48 PM
The "S Meter" on an FM rig is virtually useless. Because of the characteristics of an FM receiver, the limiters and high gain IF amplifiers will result in almost any signal that is readable, causing the meter to go nearly full scale! That is probably the main reason that almost all the antennas appear to be working the same for you. You are receiving a powerful signal from the repeater, and your receiver is giving you nearly a fully quieted signal, regardless of which antenna you have hooked up to it.

I am willing to bet, however, that you are getting into the repeater with varying degrees of noise and "scratch" and probably don't even realize it! You can "kerchunk" a repeater with a very low level signal, but a person listening on the output might not be able to understand a word you are saying if you try to talk to him!

JMHO 73, Jim

ZL3GSL
10-20-2009, 01:44 AM
It might be interesting to add another test. Try receiving with no antenna connected (just receiving, of course ;) ) . Compare holding the set, and having it standing on a table.

You might also find that having the "real" antennas close, vertical, horizontal, may affect the received signal, as they can act as passive repeaters.

AG3Y
10-20-2009, 01:56 AM
A friend and I did a similar test. I had an old HTX 202 and he had one of the modern super-compact HTs ( can't remember exactly which one , sorry ) On several repeaters, my HT was full quieting receive, while his had quite a bit of noise on the same signal! We were both running "rubber ducks" but mine is about 8 inches long, and his was probably only half the length, to go along with the sub-compact design of the HT.

I don't know if it was the antennas , or just the superior receiver of an older rig, but there WAS a difference !

With VHF/UHF operation, having your antenna high and clear of surroundings is probably the most important way to improve your performance !

73, Jim

VK6ZGO
10-20-2009, 05:23 AM
I doubled on this one --sorry

VK6ZGO

VK6ZGO
10-20-2009, 05:26 AM
Remember:
Itty bitty antennas (such as the stub ducks) will only transfer a smidgen of what rf power huge behemoth antennas (like a full-wave loop or log periodic for 160m) will, no matter the losses from impedance or design frequency mismatch. And it's kinda impossible to recover more power than the transmitter is actually emitting, if they output 1w to a 90dBi dish pointed at your 90dBi dish all you can get is 1w and that is under perfect conditions that do not exist in reality That and antennas apparently re-radiate some of the rf power they receive, so you only get a portion of the total available signal.

But always remember:
"Rf gatta go somewhere!"

BRAVO!! This point should be repeated at every opportunity,as there are a lot

of supposedly knowledgeable hams that don't understand!!!

(This is my hobbyhorse)


73 VK6ZGO

KL7AJ
10-20-2009, 07:07 PM
What you want do do is use a step attenuator (or a series of fixed attenuators) to bring the BEST antenna to just above the noise floor. Then leaving the attenuator in place, check the other antennas. This is where you'll see the difference....and I suspect it will be huge.

eric

K7JBQ
10-20-2009, 09:45 PM
Testiment for the Diamond 320A:

Checked into the CARA Monday night net using the TH-F6A a few weeks ago with the Diamond. Net control had to ask for my call three times before he confirmed it, which I thought was a tad unusual.

Until I noticed that somehow I had switched power to "EL:" a whopping 0.05 watt.

Somehow, don't think I would have made the trip with the stock antenna.

73,
Bill

K8JD
10-21-2009, 01:02 AM
If your measure of antenna capture is Quieting, there are other factors.
The nature of the multiband V/UHF recievers is a wide front end that may be desensitized by strong out of band signals that you will not even hear, such as local TV and FM transmitters or high power VHF/UHF paging transmitters.
Some of the most frustrating attempts to use a 2M rig on a high office building's roof comes from the super strong signals from nearby non-ham transmitters, they almost blocked all reception on 2M for me.
This could be your problem, where the best antenna could cause you the worse 2M reception.

K4EEZ
10-21-2009, 07:52 AM
1 on vhf/uhf height is everything.
2 a working out door antenna will out preform any indoor antenna.
3 when installing you roof antenna make sure you have screwed all the elements together and securely.

tips:
watch out for power lines, when you and friends are installing the antenna/mast, if the antenna/mast falls, let it go, and stand clear
if it falls on power lines Do not touch it or go near it, call the power company and let them shut the power off.

make sure you run a #10 AWG ground wire from the bottom of the mast to your ground pole that should be a 9ft rod banged in to the ground and bond that rod via a #10 AWG wire to you house hold service ground so all grounds are connected.

do not over tighten the rf/coax connectors

what type of coax are you going to use and how long is it going to be?
i would suggest LMR 400 for low loss on uhf

good luck
73

K4EEZ
11-30-2009, 02:10 AM
what happened to your antenna install and what was the out come????

WA9SVD
11-30-2009, 03:48 AM
If you are testing all those antennas to see how well they work on a local repeater, then they will all work very well, and you probably won't notice a difference.

How about trying the roof antenna and compare the others with a repeater that is a quite a bit ferther away or simplex and see what you get for a difference?

~~EDIT~~

I forgot you said you are testing 70 cm. I never been on 70 cm so I don't know if there are repeaters there or not.

Almost NO "testing" using a repeater (whether local or distant) is meaningful in any way. The only meaningful (albeit subjective) testing HAS to be on simplex, with local, distant, and hopefully "in between" stations. THAT will give a relative indication as to actual performance (and signal strength at various distances) of the different antennas.

I can't count the numner of times I've heard a station ask for a signal report through a repeater and get a reply "you're full scale on my meter, but you're about 90% scratch." OF COURSE the "signal" is full scale: THAT is the strength of the REPEATER into the receiving station. The "90% scratch" means the transmitting station is very, very marginal INTO the repeater.

ad: WarrenG-1