Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: Prev 1 [2]   Go Down
  Print  
Author Topic: Compact indoor antenna?  (Read 8147 times)
JOCONN
Member

Posts: 11




Ignore
« Reply #15 on: November 09, 2012, 09:34:13 AM »

He asks for the rain gutter, I understand? That is according to me not so good idea. Metal joints often give harmonics and intermodulation due to rectifying effects. It will be not isolated, and when the metal is not copper but zinc or iron, the losses are high.

But with a tuner you can get your power in it; adjustment dependent on the weather.
73 Bob


I don`t completely understand your answer, Bob. Will it work with a tuner?
Logged
K3STX
Member

Posts: 1000




Ignore
« Reply #16 on: November 09, 2012, 10:32:35 AM »

A dummy load or a lightbulb will work as an antenna. A lousy antenna, but an antenna nonetheless.

The issue is not whether it will "work", the issue is whether it will work WELL. A metal raingutter will work; not well but it will work. A WIRE outside your house, going to some sort of support (like a tree, a mast, a chimney, your NEIGHBORS chimney, your garage, etc.), will work; probably not so bad. I made LOADS of contacts between NC and Europe with 100 watts when I was in college using a wire thrown out my 2nd floor apartment window up ON TOP of the roof of the apartment. It was a two story apartment building, I reached out the window  and THREW some magnet wire with a big NUT at the end (as a weight) onto my roof. Total wire length about 40 feet. Counterpoise in apartment around baseboard. It stayed up there all winter, it was essentially invisible.

It was not a "good" antenna, but it was decent enough to have fun and work DX. My current 40M dipole up 50 feet is better, maybe in the "good" range. A really good 40M antenna would be a yagi.

Everything is relative. Just do SOMETHING. Sounds like you will AT LEAST need a tuner, get that and THEN begin to think about what next.

paul
Logged
N3DF
Member

Posts: 252




Ignore
« Reply #17 on: November 09, 2012, 11:22:50 AM »

It's not clear to me how much HF/CW experience the OP has.  He could have a lot of experience, but I'm guessing no.  Based on my experience:

QRP+Reduced Size Antenna+Indoors+Little/No HF/CW experience = Frustration.
Logged

Neil N3DF
NO2A
Member

Posts: 801




Ignore
« Reply #18 on: November 09, 2012, 12:20:25 PM »

I like the longwire and counterpoise idea. Get the best tuner you can afford. Indoor loops are fine,provided they can be tuned properly. Rain gutter antennas may or may not work as well on receive as transmit,only experimentation will tell.
Logged
PA0BLAH
Member

Posts: 0




Ignore
« Reply #19 on: November 09, 2012, 01:00:11 PM »



I don`t completely understand your answer, Bob. Will it work with a tuner?

Sorry, I try it again.

You can get your power in each piece of metal, even an iron bed will do.

Every conductor has an impedance at the feed point, dependent of the transmitting frequency.  That will probably not be close to 50 ohm, so you need an antenna tuner.

Every antenna tuner has a limited range of impedance matching.

Suppose the impedance is within the range, then you can get your transmitterpower in the conductor that is hooked up at the output of your antennatuner. Ground your transmitter.

Is the conductor close to the ground, and pretty short compared with the wavelength then the range of your transmitter will be bad.

So, looking at your rain gutter it will work. BUT
a rain gutter has different impedance over time and a lot of loss because it is not isolated, and rainor snow in it may even short it to ground. So during rainy weather the impedance will change, and you have to re-adjust your tuner.

Furthermore there are losses. In copper wire between isolators not much in iron pretty heavy. Rhombic antennas use iron open line as dummy. With large transmitters glowing visible in the night.
So when your antennaconductor is made of iron, you can put power in it with your tuner, but is is dissipated as heath and hardly radiated.

Finally, gutters are composed of pieces  gutter, added together with rivets or soldered. The joints have often for RF recitfying effects, and that give harmonic radiation.

Just try it, is a good advice. Also a good advice: QRP plus bad antenna plus unexperienced operator yields  a lot of frustration.
Start on 20 meter.

Bob
Logged
JOCONN
Member

Posts: 11




Ignore
« Reply #20 on: November 09, 2012, 02:17:20 PM »

I am not inexperienced. For years at my previous location I had an inverted V fed with with ladder line which worked quite well on several bands. Previously I have used straight dipoles, random wire and vertical dipoles. I have operated QRP and QRO both SSB and CW.
My point is this: I would like to get away from large outdoor antennas and want to try my hand at stealth or indoor setups. I have not gone this route before and was looking for advice from people who have. I am fully aware of the need for a highly effective antenna when working QRP. All said, I do appreciate the input.
Logged
WX7G
Member

Posts: 6134




Ignore
« Reply #21 on: November 09, 2012, 02:50:38 PM »

Well I've operated using indoor antennas in six apartments and two houses. Indoor antennas can work very well. My favorite indoor antenna at this time is a loop tuned by an MFJ-933 loop tuner. The disadvantages are narrow bandwidth and only two bands per loop. But, a reasonably efficient 3.5 MHz loop can fit inside a living room. Such a loop has a circumference of 40' and can be configured as a rectangle.
Logged
PA0BLAH
Member

Posts: 0




Ignore
« Reply #22 on: November 09, 2012, 03:59:36 PM »

Well I've operated using indoor antennas in six apartments and two houses. Indoor antennas can work very well. My favorite indoor antenna at this time is a loop tuned by an MFJ-933 loop tuner. The disadvantages are narrow bandwidth and only two bands per loop. But, a reasonably efficient 3.5 MHz loop can fit inside a living room. Such a loop has a circumference of 40' and can be configured as a rectangle.

Yes. A loop for dx bands 21 or 14 MHz is pretty small, but takes effort to collect materials and construct it. Furthermore due to the small bandwidth you need a motor remote tuned capacitor.  Split stator is less desired then butterfly, due to the fact the current goes from plates of one section via the axis to a plate of the other section, with mechanical joints. In a butterfly is is only running through one plate from one to the other section. A vacuum capacitor is more expensive.

During my lifetime a lot of things not thought of being hazardous turned out to be, rumours are RF might be, and there are limits specified by governments. When you put a 100 watt in a loop your distance have to be more then 20 feet in order to keep below that (pretty arbitrary) level. So decide for yourself and your family.

There is a compulsory registration of ham radio antennas, even  for indoor antennas, so you only can be stealth by unlawful behavior (in this country).



Bob
« Last Edit: November 09, 2012, 04:08:12 PM by PA0BLAH » Logged
JOCONN
Member

Posts: 11




Ignore
« Reply #23 on: November 09, 2012, 05:01:28 PM »

A short trip to my local DIY store and my radio junk box will get me all the parts I need to build a loop. As for legality ..... I live in a free country. Five/ten watts into a loop, dangerous?

Joe, VE9OCR.
Logged
STAYVERTICAL
Member

Posts: 875




Ignore
« Reply #24 on: November 10, 2012, 09:36:50 PM »

From a purely technical point of view, I can recommend a small magnetic loop as being the best performing indoor antenna I have ever used.
It does not require a counterpoise, and if you take care building it - will be reasonably efficient.
For example a 1 metre wide loop made from 19mm copper tube will be about 30 percent efficient on 10Mhz.
The efficiency rises quickly as you go up in frequency and drops quickly as you go lower.
It is quite possible to use it on 40m, as well but the efficiency will be around 11 percent.

I use a similar loop daily for a 40m net, and for dx work on higher frequencies.
I put it out in the garden on a camera tripod when operating and take it in when I am finished.
It is about 1.2 metres off the ground at the bottom.

First some caveats - it is efficient only if you build it with low RF resistance joints and materials.
As Bob says, a vacuum capacitor is the holy grail, followed by butterfly and then split stator capacitor types.
Do not use a wiper contact type of capacitor - it will lower your efficiency drastically.
Do not use coax braid in your circuit, use solid copper sheet or tubing to connect the capacitor to the tubes.
The coax braid is also lossy compared to solid copper sheet or tube.

If your loop is built well, it will be efficient, but very sharp to tune.
When you transmit with 100W or as the Sun heats up the loop, the frequency will change very slightly.
If you want to maintain maximum efficiency on transmission, make a motor drive for the capacitor, and include a very slow motion feature.
This is for those "top up" fine tuning situations while transmitting.

The vicinity near an efficient loop is a "hot" area for both R.F. and high voltage.
Even with 5 to 15 watts, your capacitor will have 1000 to 2000 volts across it.
Remember - the top of the tube will also have this voltage, not just the capacitor, so make sure no one touches it when transmitting.

Personally, if I could keep the loop about 10 to 15 feet away, I would feel safe with 15 watts, but for 100W I would make it 25 feet at least.
Take it from someone who's main antenna is a 1 metre wide magloop - they work well - but because of that you need to have caution.

They are also not a "set and forget" antenna.
They are more like playing a musical instrument - you need to keep an eye on your SWR meter to make sure it has not drifted out of tune.
I have found that across a typical ham band you will be able to hear signals, even when off tune, but they will be about 6 or 7 S points down.
When you remotely tune, the peak in signals/noise is very close to the transmitting sweet spot, so only small adjustments are necessary.

I mention the above, not to put you off using a magloop - they hit far above their weight in my experience.
But you need to have a commitment to the necessary operating style when using one.
It is far removed from the usual dipole/g5rv or even random wire with remote ATU style of operating.
The continual SWR monitoring is something which is not often mentioned by magloop users - but it is critical in getting good results.
You need to know what your ham personality/operating style is - if you are going to avoid frustration.

Do not be tempted to cut corners and use coax braid for the loop tube etc, you will still get contacts - but you will not know how well a loop can really perform.
Another thing to remember is that a small Magloop is directional over its frequency range, even at 4 foot off the ground and at 7Mhz!
I have found that stations broadside to the loop on 40m are barely readable both on transmit and receive.
So if you are in a net for example, stations perpindicular to the the plane of the loop will be very poor.
The average 7Mhz dipole at the typically low effective heights are almost omnidirectional - so you don't tend to see this effect.

In Summary:

1. There are only two critical areas of the loop - the capacitor and its connections to the loop tube.
2. Keep the capacitor on top of the loop - it's far more efficient.
3. Use copper tube - it's about double the efficiency of an aluminium tube ( 20mm diameter is sufficient).
4. It will operate quite happily at 4 foot off the ground - but it must be vertically oriented.
5. If you align it horizontally the same rules as a conventional dipole apply as regards height above ground.
6. A circular feedloop of 20 percent diameter of the main loop works well over a wide frequency range.
7. Remember the top of the loop has R.F. at HIGH VOLTAGE - if anyone contacts it they will get severe RF burns.
8. Capacitor blade spacing must be great enough to avoid flashover at the maximum power you will use.
9. Give yourself, family and pets enough clearance to avoid excessive R.F. exposure.
10. Remember - it's directional characteristics persist with frequency - you will need to rotate the antenna sometimes.

If you keep the reality of building and using a loop in mind, you will find it outperforms any other electrically small antenna by far.
In addition, with motorized tuning, it will give you at least 5 ham bands (and all between) with efficiency increasing exponentially as you go higher.

As in many things you need to accept some limitations, to gain the rewards - but having open eyes is better than stumbling in the dark.

Good luck es 73 - Rob
« Last Edit: November 10, 2012, 09:50:08 PM by STAYVERTICAL » Logged
PA0BLAH
Member

Posts: 0




Ignore
« Reply #25 on: November 11, 2012, 04:02:33 AM »

A short trip to my local DIY store and my radio junk box will get me all the parts I need to build a loop. As for legality ..... I live in a free country. Five/ten watts into a loop, dangerous?

Joe, VE9OCR.

Joe, When you ask a question on a forum you often get a lot of contradictional and invalid answers.
So I feel urged to tell you that what StayVertical writes above, is quite right and 100% usefull information.
Don't use coax as capacitors.

Take care
Bob

Logged
Pages: Prev 1 [2]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!