UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Amateur and Military Radio

Notices

Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

Closed Thread
 
Thread Tools
Old 30th Sep 2019, 5:20 pm   #101
G8HQP Dave
Rest in Peace
 
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,872
Default Re: Antenna recommendation for 40 and 80M bands

Quote:
as to SWR, it is often overlooked that a dipole has an impedance of 73 ohms and the coax is 50 ohm so 75/50 = near 1.5 to 1 is the best real SWR that should be expected, anything up to 2-1 is fine.
A resonant dipole in free space has an impedance of 73 ohms. Near the ground (and most amateur antennas are near the ground) it will be different, probably lower than 73 ohms.
G8HQP Dave is offline  
Old 30th Sep 2019, 9:29 pm   #102
James Duncan
Pentode
 
Join Date: May 2010
Location: Wick, Scotland, UK.
Posts: 166
Default Re: Antenna recommendation for 40 and 80M bands

Quote:
Originally Posted by G8HQP Dave View Post
Quote:
as to SWR, it is often overlooked that a dipole has an impedance of 73 ohms and the coax is 50 ohm so 75/50 = near 1.5 to 1 is the best real SWR that should be expected, anything up to 2-1 is fine.
A resonant dipole in free space has an impedance of 73 ohms. Near the ground (and most amateur antennas are near the ground) it will be different, probably lower than 73 ohms.
Yes Dave ground to antenna wire capacitance effect, depend on the type of ground and height above ground BUT does not change a great deal, many many test have revealed that decades ago but often as you say forgotten.
too much made about looking for a perfect 1-1 SWR, in reality not possible unless one uses a tuner to fake it.
James Duncan is offline  
Old 30th Sep 2019, 9:41 pm   #103
ORAWA01
Hexode
 
Join Date: Apr 2011
Location: Stirlingshire, UK.
Posts: 370
Default Re: Antenna recommendation for 40 and 80M bands

Quote:
Originally Posted by James Duncan View Post
but often as you say forgotten.
too much made about looking for a perfect 1-1 SWR, in reality not possible unless one uses a tuner to fake it.
Do you not use ATU for transmitting at all then?

In fact, under SWR 2, transceivers don't need ATU for TXing?
ORAWA01 is offline  
Old 1st Oct 2019, 3:34 am   #104
Radio Wrangler
Dekatron
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 14,771
Default Re: Antenna recommendation for 40 and 80M bands

Most will give rated power into VSWRs of about 1.5 and below. Above that you can expect them to start reducing output power. It's not purely a matter of the scalar VSWR, it also depends on the phase angle of the reflected signal at the transmitter.

One transmitter I've designed for the day job has to meet requirements to put >=50% of rated power into a 2:1 VSWR load of any angle for the US market, and >=40% of rated power into a 3:1 VSWR load of any angle for European market . Unfortunately, it's not possible to say that if you pass one requirement, you'll pass the other, so both have to be designed for and tested. For a transmitter with ALC control of power combined with active feedback linearisation, it gets rather involved and the testing involves slide lines extended by lots of different lengths of feeder. Oh, and there are specs for modulation depth, distortion and RF spectrum under all tested load conditions. I've become more familiar than I ever wanted to be with the effects of mis-loads on transmitters!

In amateur radio terms, though, your transmitter will start folding back power into worsening loads. You could fix this with an ATU, but then you get the ATU losses as well as magnified losses of the mismatched feeder run. You could fix it by changing the antenna, but this is also not lossless and may move your direction of best radiation (in 3-dimensions)

These are all interlinked compromises and any general advice might not be valid under your circumstances.

Don't overlook ATU losses. Georg Burt, GM3OXX had worked DXCC (and then some!) with a 1-watt transmitter before he discovered the ATU he was using had 3dB loss on most of the bands he used. So he'd really done it on 500mW. He got interested in low-loss ATUs.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is offline  
Old 1st Oct 2019, 9:56 am   #105
Bazz4CQJ
Nonode
 
Bazz4CQJ's Avatar
 
Join Date: Apr 2009
Location: Oxfordshire, UK.
Posts: 2,816
Default Re: Antenna recommendation for 40 and 80M bands

Quote:
Originally Posted by Radio Wrangler View Post
Don't overlook ATU losses. Georg Burt, GM3OXX had worked DXCC (and then some!) with a 1-watt transmitter before he discovered the ATU he was using had 3dB loss on most of the bands he used. So he'd really done it on 500mW. He got interested in low-loss ATUs.

David
I could be wrong, but I'm struggling to recall adverts for commercial ham-radio ATU's, or magazine articles for DIY ATU's, ever mentioning insertion loss. Nor is the term "Low-Loss ATU" something you hear very often. There seems to be much more emphasis on having ATU's which will load up on any 'wet piece of string' which is at hand.

I wonder what a set of rules would look like for the design & construction of a Low-Loss ATU? Perhaps one could start by looking inside an MFJ ATU for some ideas? .

B
__________________
Data beats opinions most times... that's my opinion, though I have no data on that.

Last edited by Bazz4CQJ; 1st Oct 2019 at 10:01 am.
Bazz4CQJ is offline  
Old 1st Oct 2019, 3:41 pm   #106
Radio Wrangler
Dekatron
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 14,771
Default Re: Antenna recommendation for 40 and 80M bands

Quote:
Originally Posted by Bazz4CQJ View Post
I could be wrong, but I'm struggling to recall adverts for commercial ham-radio ATU's, or magazine articles for DIY ATU's, ever mentioning insertion loss. Nor is the term "Low-Loss ATU" something you hear very often. There seems to be much more emphasis on having ATU's which will load up on any 'wet piece of string' which is at hand.

I wonder what a set of rules would look like for the design & construction of a Low-Loss ATU? Perhaps one could start by looking inside an MFJ ATU for some ideas? .

B
That's the herd of elephants in the room. The efficiency or lossiness of an ATU is never mentioned in adverts or constructional articles.

Most ATUs can be looked at as a resonator with adjustable couplings to feed in power from the transmitter and to feed power out to the antenna feeder.

The resonator can be adjusted to compensate for reactive components of the antenna impedance.

The Q of the resonator depends on the coupling settings. If you tune it with light coupling the Q can be rather high. This means that the tuning is very sharp, and on receive it acts as a preselector filter handily removing large signals a bit off your frequency. However, this setting multiplies the losses of the coils, capacitors and wiring in the ATU. You get a high-loss ATU at these settings. However, the high-Q settings of the ATU are forced when you want to transform a somewhat high or somewhat low impedance to the 50Ohms your transmitter would like to see.

With stronger coupling, the antenna and the transmitter load the ATU resonator more. Losses are less and the ATUs ability to match extreme impedances is curtailed.

So ATU loss varies widely with what impedance you are using it to match.

Measuring the loss of an ATU is not easy, you need to be able to measure true power into whatever impedance it is transforming. Most power meters are dedicated to the 50 Ohm world and are useless here. You also have to handle phase shifted current and voltage and not be thrown out by power factor.

In most ATUs the inductor is the big culprit, and its Q can be spoiled by putting it in too small a box (Whoosh! there went most of the MFJ, Trio, Yaesu and Icom atus)

The power rating of ATUs is equally hilarious. Our Atlantically-challenged friends like talking of 'a full kW tuner' but they aren't power limited.... they are voltage limited and current limited. Tune 'em up for high Q and the wattage those limits translate into comes crashing down. Oddly, their best power rating is when they are transforming a 50 Ohm antanna into 50 Ohms to present to the transmitter.... um, just when you don't actually need the ATU.

When you are using an ATU to do some real transformation, the power rating will have fallen, and its losses will have risen. The universe seems rather unfair at times like these.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is offline  
Old 1st Oct 2019, 8:16 pm   #107
James Duncan
Pentode
 
Join Date: May 2010
Location: Wick, Scotland, UK.
Posts: 166
Default Re: Antenna recommendation for 40 and 80M bands

Quote:
Originally Posted by budkor22 View Post
Quote:
Originally Posted by James Duncan View Post
but often as you say forgotten.
too much made about looking for a perfect 1-1 SWR, in reality not possible unless one uses a tuner to fake it.
Do you not use ATU for transmitting at all then?

In fact, under SWR 2, transceivers don't need ATU for TXing?
No, I do not use a tuner, I make sure the antennas I have in use are resonant mid band, no balun,
as to coax just avoid 1/4 wavelength multiples in total length.
If you use a vertical loop antenna ( triangle)the impedance will be high but a quarter wave length of 75 ohm between the antenna centre and the 50 ohm coax will take it down to near 50 ohms, again no tuner required, use of coax stubs and in line lengths of 75 ohm coax has sort of been forgotten and few know of this loss free application.
MM0HDW
James Duncan is offline  
Old 1st Oct 2019, 8:28 pm   #108
James Duncan
Pentode
 
Join Date: May 2010
Location: Wick, Scotland, UK.
Posts: 166
Default Re: Antenna recommendation for 40 and 80M bands

Quote:
Originally Posted by Radio Wrangler View Post
Quote:
Originally Posted by Bazz4CQJ View Post
I could be wrong, but I'm struggling to recall adverts for commercial ham-radio ATU's, or magazine articles for DIY ATU's, ever mentioning insertion loss. Nor is the term "Low-Loss ATU" something you hear very often. There seems to be much more emphasis on having ATU's which will load up on any 'wet piece of string' which is at hand.

I wonder what a set of rules would look like for the design & construction of a Low-Loss ATU? Perhaps one could start by looking inside an MFJ ATU for some ideas? .

B
That's the herd of elephants in the room. The efficiency or lossiness of an ATU is never mentioned in adverts or constructional articles.

Most ATUs can be looked at as a resonator with adjustable couplings to feed in power from the transmitter and to feed power out to the antenna feeder.

The resonator can be adjusted to compensate for reactive components of the antenna impedance.

The Q of the resonator depends on the coupling settings. If you tune it with light coupling the Q can be rather high. This means that the tuning is very sharp, and on receive it acts as a preselector filter handily removing large signals a bit off your frequency. However, this setting multiplies the losses of the coils, capacitors and wiring in the ATU. You get a high-loss ATU at these settings. However, the high-Q settings of the ATU are forced when you want to transform a somewhat high or somewhat low impedance to the 50Ohms your transmitter would like to see.

With stronger coupling, the antenna and the transmitter load the ATU resonator more. Losses are less and the ATUs ability to match extreme impedances is curtailed.

So ATU loss varies widely with what impedance you are using it to match.

Measuring the loss of an ATU is not easy, you need to be able to measure true power into whatever impedance it is transforming. Most power meters are dedicated to the 50 Ohm world and are useless here. You also have to handle phase shifted current and voltage and not be thrown out by power factor.

In most ATUs the inductor is the big culprit, and its Q can be spoiled by putting it in too small a box (Whoosh! there went most of the MFJ, Trio, Yaesu and Icom atus)

The power rating of ATUs is equally hilarious. Our Atlantically-challenged friends like talking of 'a full kW tuner' but they aren't power limited.... they are voltage limited and current limited. Tune 'em up for high Q and the wattage those limits translate into comes crashing down. Oddly, their best power rating is when they are transforming a 50 Ohm antanna into 50 Ohms to present to the transmitter.... um, just when you don't actually need the ATU.

When you are using an ATU to do some real transformation, the power rating will have fallen, and its losses will have risen. The universe seems rather unfair at times like these.

David
Well said David
All the more reason to use resonant antennas, I just do not understand why skilled amateurs mess about with making or buying antennas that require add ons to fool the rig, expensive and playing to hands of the antenna sellers who at times seem to be reinventing the wheel ( to a square format) inducing people to buy tuners, baluns etc.
make the antenna correct and you will get max power into the antenna MM0HDW
James Duncan is offline  
Old 1st Oct 2019, 8:44 pm   #109
Bazz4CQJ
Nonode
 
Bazz4CQJ's Avatar
 
Join Date: Apr 2009
Location: Oxfordshire, UK.
Posts: 2,816
Default Re: Antenna recommendation for 40 and 80M bands

I must not miss an opportunity to bring up the subject of GDO's here .

I'm hoping to put up an inverted V for 80m soon, but it will be compromised by the fact that I cannot get the two legs in straight line; they will probably be at an angle of 120' and each will be in a rather different environment.

So, what would be the best way to check for resonance with that? The idea which occurred to me was to check each leg independently using a GDO and trim to get them both on the same frequency.

Does that sound reasonable?

B
__________________
Data beats opinions most times... that's my opinion, though I have no data on that.
Bazz4CQJ is offline  
Old 1st Oct 2019, 11:10 pm   #110
ORAWA01
Hexode
 
Join Date: Apr 2011
Location: Stirlingshire, UK.
Posts: 370
Default Re: Antenna recommendation for 40 and 80M bands

Quote:
Originally Posted by Bazz4CQJ View Post
I must not miss an opportunity to bring up the subject of GDO's here .

I'm hoping to put up an inverted V for 80m soon, but it will be compromised by the fact that I cannot get the two legs in straight line; they will probably be at an angle of 120' and each will be in a rather different environment.

So, what would be the best way to check for resonance with that? The idea which occurred to me was to check each leg independently using a GDO and trim to get them both on the same frequency.

Does that sound reasonable?

B
Can GDO check wire antenna's resonance? I used to think they are for checking inductors and coils.

Saying that, when I see my GDO, it has input for a wire antenna. Was wondering what that was for.
ORAWA01 is offline  
Old 1st Oct 2019, 11:52 pm   #111
Bazz4CQJ
Nonode
 
Bazz4CQJ's Avatar
 
Join Date: Apr 2009
Location: Oxfordshire, UK.
Posts: 2,816
Default Re: Antenna recommendation for 40 and 80M bands

Yes, a GDO was very much the way things were done before the modern 'antenna analysers' came along. The page shown below is taken from the manual for the American "Megacycle Meter", which was in widespread field use with the US Army, but I'm sure you'll find many other online references for using a GDO with antennas.

B
Attached Files
File Type: pdf Model 59 green manual.pdf (1.08 MB, 12 views)
__________________
Data beats opinions most times... that's my opinion, though I have no data on that.
Bazz4CQJ is offline  
Old 1st Oct 2019, 11:53 pm   #112
Radio Wrangler
Dekatron
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 14,771
Default Re: Antenna recommendation for 40 and 80M bands

Some GDOs have a simple antenna input and allow the oscillator to be switched off. In this way you can use them as a simple absorption wavemeter, or as a modulation monitor into a pair of headphones (AM of course) or as a field strength monitor. Handy if you have no other RF test equipment.

You can put a coupling coil on the antenna feeder and dip that (interpretation can be a bit difficult) to get an idea of antenna resonance.

As far as exhortations to eschew ATUs and have resonant antennae go... I have room for one HF antenna, and it needs to be rather stealthy. It cannot be resonant on all the bands I wish to use, nor can it be any sort of optimum on any of them. Consequently I have a wire antenna as large as fits in my property and I have no choice but to use an ATU. If I'd cut it for one band, I might have got one of the harmonic bands in, but I'd still have needed the ATU for the other bands.

Knowing I'm going to have to use it off-resonance on at least some of the HF bands, I have made sure the feeder is very low loss. Operation away from the Zo of a feeder tends to multiply losses, so start with low loss and the net product of loss is less.

The antenna and feeder are balanced. This nulls stray radiation from the feeder, nulls the ingress of local noise and confers these advantages at all frequencies. This also means that I am not dependent on having a good ground. In fact I want to galvanically isolate my antenna from the house ground because of PME complications.

So I need some sort of balun. Baluns are difficult to make broadband, and are very difficult to make work over a range of impedances. Needing a wide range of impedances AND full HF coverage puts them firmly in the impossible category unless high losses can be tolerated. The solution is to have a balanced resonator in the ATU and to have a coupling coil to give galvanic isolation. The transmitter coupling can be unbalanced, and the antenna coupling can be balanced... so my ATU performs the needed balancing act and at the same time performs the ATU function.

There is plenty of space out at the stables... room to put up rhombics if wanted, but that's miles away from home. The radio club with its tribander on a tower etc is closer. So yes, I could have resonant dipoles If I wished, but doing it at home so I can have a shufti on HF while at home involves compromise. The choice of the right ATU makes that compromise work for me.

If I limited myself to resonant antennae only and no ATU, I would as a consequence be limited in which bands I could use... limited mostly to the bands which are closed through sunspot minima.

George Burt scored over 300 countries with 1W into a tuner with 3dB loss into an open wire feeder and a dipole set by the spacing of two chimney stacks. Proof enough it can work, but I'm not an operator in his league.

But, everyone gets to make their own choice. That's freedom for you!

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is offline  
Old 2nd Oct 2019, 7:18 am   #113
ORAWA01
Hexode
 
Join Date: Apr 2011
Location: Stirlingshire, UK.
Posts: 370
Default Re: Antenna recommendation for 40 and 80M bands

Thank you for the great info and the attachment.

I was also wondering what the headphone jack was on my GDO. It is the one made by LOWE FX-1. I must locate the manual which gone missing.

For ATU, which ones are low loss ATUs from commercial ones? Or would it be the ones made homebrew?
ORAWA01 is offline  
Old 2nd Oct 2019, 10:41 am   #114
ORAWA01
Hexode
 
Join Date: Apr 2011
Location: Stirlingshire, UK.
Posts: 370
Default Re: Antenna recommendation for 40 and 80M bands

Another problem with tuned dipole antennas are, that they work properly when they are at least half wave up from the ground. It is a bit challenge to achieve for most town dwellers. Hence opting for other compromises like this one, or multband long wire with ATU?

https://www.youtube.com/watch?v=Al3ZJipFq6k
ORAWA01 is offline  
Old 2nd Oct 2019, 11:04 am   #115
G8HQP Dave
Rest in Peace
 
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,872
Default Re: Antenna recommendation for 40 and 80M bands

Quote:
Originally Posted by Bazz4CQJ
The idea which occurred to me was to check each leg independently using a GDO and trim to get them both on the same frequency.
No, that won't work - unless you have a good RF ground up in the sky. You need to check the resonance of the whole antenna. How would you measure a resistor if you only had access to one leg?
G8HQP Dave is offline  
Old 2nd Oct 2019, 12:49 pm   #116
Bazz4CQJ
Nonode
 
Bazz4CQJ's Avatar
 
Join Date: Apr 2009
Location: Oxfordshire, UK.
Posts: 2,816
Default Re: Antenna recommendation for 40 and 80M bands

Quote:
Originally Posted by G8HQP Dave View Post
No, that won't work - unless you have a good RF ground up in the sky. You need to check the resonance of the whole antenna. How would you measure a resistor if you only had access to one leg?
I see your point, and that's why I posed the question. Perhaps I need to clarify. What I was thinking of was to test each leg, treating as a longwire, and coupling directly to the end of it, not via feeder.

Does your argument imply that you could not resonate a longwire antenna with a GDO, but only a dipole?

But, in terms of erecting an inverted V where you know the two legs are not in perfect free-space, but are affected by quite different surroundings (trees, buildings), maybe you just have to proceed by trimming each leg to the same length?

B
__________________
Data beats opinions most times... that's my opinion, though I have no data on that.
Bazz4CQJ is offline  
Old 2nd Oct 2019, 3:21 pm   #117
G8HQP Dave
Rest in Peace
 
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,872
Default Re: Antenna recommendation for 40 and 80M bands

To couple a GDO to an antenna you need a small sense coil. For a dipole it is obvious that you connect the coil at the feedpoint; you have two antenna terminals and two coil ends. For a monopole you have a wire with one terminal at the feedpoint and a coil with two ends, so you connect one end of the coil to the antenna end. What do you connect the other end of the coil to?

Have you noticed that a 12V power supply always has both a 12V terminal and a 0V terminal? How much power could you draw if you use just the 12V terminal?

The two sides of a dipole, whether bent or not, do not have to be exactly the same length or be in exactly the same environment. The main advantage of good balance is that it can reduce local noise pickup by the antenna itself. An off-centre fed dipole (OCFD) may need a better balun than a centre-fed dipole. Some people deliberately use an OCFD to get better multiband performance.
G8HQP Dave is offline  
Old 2nd Oct 2019, 3:35 pm   #118
G6Tanuki
Dekatron
 
G6Tanuki's Avatar
 
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 8,671
Default Re: Antenna recommendation for 40 and 80M bands

RF-on-the-coax [well, some of it] is not always a bad thing - it can actually be used to generate a vertically-polarised component to the radiated signal in addition to the horizontally-polarised component from the dipole proper.

Known as a "Controlled Feeder Radiation" dipole - see, for example,

https://www.robkalmeijer.nl/techniek...e46/index.html

https://www.radioenthusiast.co.uk/ne...al-cfr-dipole/
G6Tanuki is offline  
Old 2nd Oct 2019, 5:13 pm   #119
G8HQP Dave
Rest in Peace
 
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,872
Default Re: Antenna recommendation for 40 and 80M bands

There is a world of difference between deliberately using a controlled section of feeder outer current (stopped by a CM choke at the desired place), and hoping for the best by omitting a balun from a normal dipole with coax feed.
G8HQP Dave is offline  
Old 2nd Oct 2019, 6:14 pm   #120
Bazz4CQJ
Nonode
 
Bazz4CQJ's Avatar
 
Join Date: Apr 2009
Location: Oxfordshire, UK.
Posts: 2,816
Default Re: Antenna recommendation for 40 and 80M bands

Quote:
Originally Posted by G8HQP Dave View Post
To couple a GDO to an antenna you need a small sense coil. For a dipole it is obvious that you connect the coil at the feedpoint; you have two antenna terminals and two coil ends. For a monopole you have a wire with one terminal at the feedpoint and a coil with two ends, so you connect one end of the coil to the antenna end. What do you connect the other end of the coil to?

Have you noticed that a 12V power supply always has both a 12V terminal and a 0V terminal? How much power could you draw if you use just the 12V terminal?
Firstly, I think that everyone on this forum knows that you are a highly qualified engineer, professionally involved in radio, whereas I have declared on a number of occasions that I am an "amateur amateur", who passed the RAE back in 1968 and hasn't been very serious about the hobby since then. I think that with the remarks about 12V psu's, you just let yourself down.

I have never used an end-fed antenna for transmitting, but I assume that to use a GDO or any kind of antenna analyser you would have to inductively couple it in to a circuit with a ground or a counterpoise.

At that point, it could be the case that you are simply going back to towards taking measurements on the actual inverted-V rather than its 'components'. So, from what I can see with my limited expertise, it seems likely that even if you know the two legs of the V are in different environments, trying to do anything to optimise them could be hard work. But that's why I asked the question, on this hobbyist forum, thinking perhaps that the clever guys might have a card up their sleeve.

B
__________________
Data beats opinions most times... that's my opinion, though I have no data on that.
Bazz4CQJ is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 2:41 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2020, vBulletin Solutions, Inc.
Copyright ©2002 - 2020, Paul Stenning.