Advice


John McCarthy
 

Hi all I was fortunate to win a 1.2m offset dish with flat roof type stand on Ebay for just 99p !!! I'd like to use this for portable outings and I've found that a 2" heavy duty swaged aluminium pole I have fits over the pole on the base and will act as a extension pole. I intend to make a wooden bush up to fit in the base pole and grease it to aid turning the pole, with a clamp fixed to the roof rack of the car to lock the dish in position. What I'd like some advice on is the feeding of the dish, I'd like to use it on 23cm, 13cm, 9cm and 3cm. I intend to have all the transverters in one box and a single IF with each transverter switch in as required. Now Ideally to have one feed to cover all the bands would be ideal but I'm guessing that's not go'na be the case, so if I ran a length of LDF-250 from the back of the dish to the feed and used N type plugs so I could just swap the feeds for each band am I likely to run into any problems at 3cm? LDF250 is spec'd to 13ghz and I would think I'd only need about 1.5m . Also does anyone else use this size dish portable if so how do you use it ?


cheers John G7JTT


hmyjr@...
 

An eBay bargain!

Others on this group will know about multiple feeds & working portable with a dish etc.

I've tested what look like some decent bits of semi-rigid coax at 3cm & have found a lot of variation in the losses. Half the power can disappear in quite short lengths, maybe 20 or 30cm (from memory - my memory!). So 1.5 m might be a decent dummy load!

But where there is a will, there is a way.

All the best with the project John.

73 Graham - G3YJR


Gordon REASON <gordonj.reason@...>
 

Test the longest length of  semi-ridid , and note the loss .

Put the lengths of semi-rigid into a low oven for a few hours ........ 80 to 100 C ?

Allow to cool ...........then retest .

enjoy with a glass of fine wine .....


On 12 May 2018 at 10:12 "hmyjr@... [ukmicrowaves]" <ukmicrowaves@...> wrote:

 

An eBay bargain!

Others on this group will know about multiple feeds & working portable with a dish etc.

I've tested what look like some decent bits of semi-rigid coax at 3cm & have found a lot of variation in the losses. Half the power can disappear in quite short lengths, maybe 20 or 30cm (from memory - my memory!). So 1.5 m might be a decent dummy load!

But where there is a will, there is a way.

All the best with the project John.

73 Graham - G3YJR


 

 


hmyjr@...
 

What does this do Gordon? Drive out moisture?

The wine makes sense though.

Graham - G3YJR


John McCarthy
 

Thanks to those who have replied I'm guessing I'll just have to test the length of 250 and just see, but guess the losses may be too high. I'd still like to know if anyone else uses a dish this size portable and how they have it set up and any multi band feeds ?

cheers John 


John McCarthy
 

just came across this site which has a multi band ring feed http://gvnweb.ddns.net:8081/dish1.html and links to some design software as well. the only thing is this is used on a prime focus dish are these feeds ok to use with an off set dish ?

cheers John 


Christopher Bartram <cbartram@...>
 

Hello John

In a word, I'd suggest no.

The 'ring' feed will have too great a beamwidth to feed an offset dish with a 0.6 - 0.7f/D ratio efficiently. That will result in spillover and the resulting waste of power (or poor noise temperature on receive) due to the generation of sidelobes.

There are few compact dish feeds which will feed an offset TV dish efficiently on the bands below about  5GHz. Making an efficient multiband feed is complicated by the tendency of the phase-centre of the feed to vary with frequency.

Some dual-band feeds seem to work reasonably well (eg.W1GHZ) but they are usually a compromise due to the effective aperture (and thus beamwidth, and thus effective efficiency, of the feed) and phase centre differing from one band to the other.

Vy 73

Chris

G4DGU


On 13/05/18 13:43, g7jtt@... [ukmicrowaves] wrote:
just came across this site which has a multi band ring feed http://gvnweb.ddns.net:8081/dish1.html and links to some design software as well. the only thing is this is used on a prime focus dish are these feeds ok to use with an off set dish ?

cheers John 



John McCarthy
 

Ok thanks Chris, it's looking more and more likely that I'll have to make separate feeds and just have to swap to change band. It started off as a nice easy option for multi band portable use but think I'll have to reassess my plans.

Cheers John 

On Sunday, 13 May 2018, 14:11:25 BST, Christopher Bartram cbartram@... [ukmicrowaves] wrote:


 

Hello John

In a word, I'd suggest no.

The 'ring' feed will have too great a beamwidth to feed an offset dish with a 0.6 - 0.7f/D ratio efficiently. That will result in spillover and the resulting waste of power (or poor noise temperature on receive) due to the generation of sidelobes.

There are few compact dish feeds which will feed an offset TV dish efficiently on the bands below about  5GHz. Making an efficient multiband feed is complicated by the tendency of the phase-centre of the feed to vary with frequency.

Some dual-band feeds seem to work reasonably well (eg.W1GHZ) but they are usually a compromise due to the effective aperture (and thus beamwidth, and thus effective efficiency, of the feed) and phase centre differing from one band to the other.

Vy 73

Chris

G4DGU


On 13/05/18 13:43, g7jtt@... [ukmicrowaves] wrote:
just came across this site which has a multi band ring feed http://gvnweb.ddns.net:8081/dish1.html and links to some design software as well. the only thing is this is used on a prime focus dish are these feeds ok to use with an off set dish ?

cheers John 



KENT BRITAIN
 

Log Periodics do work well as a dish feed.
I agree, not quite as well as an optimized scalar feed, but the difference
is about 1 dB on a small dish.  And you get 3 or 4 ham bands.

The feedpoint of a dish is a theoretical point and usually calculated as though the
dish is used at optical frequencies.    In testing well over 100 dishes, the optimal
position for the feed is almost always closer to the dish than the calculated feed point.
In part this is because the feed is not an infinitesimal point.

Yes, the phase center of a log periodic does move, but it doesn't move very much.
When optimized at the highest frequency of use, the phase error at the lowest
frequency of use is quite minor.

Another option is using a Vivaldi antenna for the feed.  I like to think of a Vivaldi
and a Ridged Horn without the horn.    Phase center is pretty constant with
frequency.   The Vivaldi has about 2 dB more gain than a Log Periodic and would
be more suitable for most offset fed dishes.

73  G8EMY/W5



From: "g7jtt@... [ukmicrowaves]"
To: ukmicrowaves@...
Sent: Sunday, May 13, 2018 7:44 AM
Subject: [ukmicrowaves] Re: Advice

 
just came across this site which has a multi band ring feed http://gvnweb.ddns.net:8081/dish1.html and links to some design software as well. the only thing is this is used on a prime focus dish are these feeds ok to use with an off set dish ?

cheers John 



John McCarthy
 

Hi Kent thanks for a very informative reply I have one of your log periodic pcb antenna's so will give that a try. I see G4DDK does not stock your 1.5 to 15Ghz Vivaldi antenna, is there any likely hood of this changing ? If not will need to order directly . Also what is the power handling of the antenna's as I have around 50w on 23cm and 13cm whilst only 2w on 3cm.

All best John


On Sun, 13 May 2018 at 15:44, KENT BRITAIN WA5VJB@... [ukmicrowaves]
wrote:
 

Log Periodics do work well as a dish feed.
I agree, not quite as well as an optimized scalar feed, but the difference
is about 1 dB on a small dish.  And you get 3 or 4 ham bands.

The feedpoint of a dish is a theoretical point and usually calculated as though the
dish is used at optical frequencies.    In testing well over 100 dishes, the optimal
position for the feed is almost always closer to the dish than the calculated feed point.
In part this is because the feed is not an infinitesimal point.

Yes, the phase center of a log periodic does move, but it doesn't move very much.
When optimized at the highest frequency of use, the phase error at the lowest
frequency of use is quite minor.

Another option is using a Vivaldi antenna for the feed.  I like to think of a Vivaldi
and a Ridged Horn without the horn.    Phase center is pretty constant with
frequency.   The Vivaldi has about 2 dB more gain than a Log Periodic and would
be more suitable for most offset fed dishes.

73  G8EMY/W5



From: "g7jtt@... [ukmicrowaves]"
To: ukmicrowaves@...
Sent: Sunday, May 13, 2018 7:44 AM
Subject: [ukmicrowaves] Re: Advice

 
just came across this site which has a multi band ring feed http://gvnweb.ddns.net:8081/dish1.html and links to some design software as well. the only thing is this is used on a prime focus dish are these feeds ok to use with an off set dish ?

cheers John 



KENT BRITAIN
 

Since I gave Sam an handful just a few weeks ago, he should have the 1.5-15's.
And at reduced performance they do work down to 800 MHz.

Wish I had enough power to test them to destruction.
50 watts of SSB or CW should be fine.   FM or one of the near CW digital modes
would be close, sort of depends on the length of your feedline.

BUT, it you cook it, let me know at what power level, and I will get you another one.
Good data point.



From: "'g7jtt@...' g7jtt@... [ukmicrowaves]"
To: "ukmicrowaves@..."
Sent: Sunday, May 13, 2018 11:22 AM
Subject: Re: [ukmicrowaves] Re: Advice

 
Hi Kent thanks for a very informative reply I have one of your log periodic pcb antenna's so will give that a try. I see G4DDK does not stock your 1.5 to 15Ghz Vivaldi antenna, is there any likely hood of this changing ? If not will need to order directly . Also what is the power handling of the antenna's as I have around 50w on 23cm and 13cm whilst only 2w on 3cm.

All best John


On Sun, 13 May 2018 at 15:44, KENT BRITAIN WA5VJB@... [ukmicrowaves]
wrote:
 
Log Periodics do work well as a dish feed.
I agree, not quite as well as an optimized scalar feed, but the difference
is about 1 dB on a small dish.  And you get 3 or 4 ham bands.

The feedpoint of a dish is a theoretical point and usually calculated as though the
dish is used at optical frequencies.    In testing well over 100 dishes, the optimal
position for the feed is almost always closer to the dish than the calculated feed point.
In part this is because the feed is not an infinitesimal point.

Yes, the phase center of a log periodic does move, but it doesn't move very much.
When optimized at the highest frequency of use, the phase error at the lowest
frequency of use is quite minor.

Another option is using a Vivaldi antenna for the feed.  I like to think of a Vivaldi
and a Ridged Horn without the horn.    Phase center is pretty constant with
frequency.   The Vivaldi has about 2 dB more gain than a Log Periodic and would
be more suitable for most offset fed dishes.

73  G8EMY/W5



From: "g7jtt@... [ukmicrowaves]" <ukmicrowaves@...>
To: ukmicrowaves@...
Sent: Sunday, May 13, 2018 7:44 AM
Subject: [ukmicrowaves] Re: Advice

 
just came across this site which has a multi band ring feed http://gvnweb.ddns.net:8081/dish1.html and links to some design software as well. the only thing is this is used on a prime focus dish are these feeds ok to use with an off set dish ?

cheers John 





John McCarthy
 

Ok I'll contact Sam and see if I can purchase one, as to cooking the antenna it's likely to be used on SSB and some DATV so I'll let you know hi. Once again thanks for the advice.

Cheers John


On Sun, 13 May 2018 at 17:37, KENT BRITAIN WA5VJB@... [ukmicrowaves]
wrote:
 

Since I gave Sam an handful just a few weeks ago, he should have the 1.5-15's.
And at reduced performance they do work down to 800 MHz.

Wish I had enough power to test them to destruction.
50 watts of SSB or CW should be fine.   FM or one of the near CW digital modes
would be close, sort of depends on the length of your feedline.

BUT, it you cook it, let me know at what power level, and I will get you another one.
Good data point.



From: "'g7jtt@...' g7jtt@... [ukmicrowaves]"
To: "ukmicrowaves@..."
Sent: Sunday, May 13, 2018 11:22 AM
Subject: Re: [ukmicrowaves] Re: Advice

 
Hi Kent thanks for a very informative reply I have one of your log periodic pcb antenna's so will give that a try. I see G4DDK does not stock your 1.5 to 15Ghz Vivaldi antenna, is there any likely hood of this changing ? If not will need to order directly . Also what is the power handling of the antenna's as I have around 50w on 23cm and 13cm whilst only 2w on 3cm.

All best John


On Sun, 13 May 2018 at 15:44, KENT BRITAIN WA5VJB@... [ukmicrowaves]
wrote:
 
Log Periodics do work well as a dish feed.
I agree, not quite as well as an optimized scalar feed, but the difference
is about 1 dB on a small dish.  And you get 3 or 4 ham bands.

The feedpoint of a dish is a theoretical point and usually calculated as though the
dish is used at optical frequencies.    In testing well over 100 dishes, the optimal
position for the feed is almost always closer to the dish than the calculated feed point.
In part this is because the feed is not an infinitesimal point.

Yes, the phase center of a log periodic does move, but it doesn't move very much.
When optimized at the highest frequency of use, the phase error at the lowest
frequency of use is quite minor.

Another option is using a Vivaldi antenna for the feed.  I like to think of a Vivaldi
and a Ridged Horn without the horn.    Phase center is pretty constant with
frequency.   The Vivaldi has about 2 dB more gain than a Log Periodic and would
be more suitable for most offset fed dishes.

73  G8EMY/W5



From: "g7jtt@... [ukmicrowaves]"
To: ukmicrowaves@...
Sent: Sunday, May 13, 2018 7:44 AM
Subject: [ukmicrowaves] Re: Advice

 
just came across this site which has a multi band ring feed http://gvnweb.ddns.net:8081/dish1.html and links to some design software as well. the only thing is this is used on a prime focus dish are these feeds ok to use with an off set dish ?

cheers John 





SAM JEWELL
 

Now dealing with it....

Sam

On Sunday, May 13, 2018, 5:37 pm, KENT BRITAIN WA5VJB@... [ukmicrowaves] wrote:

 

Since I gave Sam an handful just a few weeks ago, he should have the 1.5-15's.
And at reduced performance they do work down to 800 MHz.

Wish I had enough power to test them to destruction.
50 watts of SSB or CW should be fine.   FM or one of the near CW digital modes
would be close, sort of depends on the length of your feedline.

BUT, it you cook it, let me know at what power level, and I will get you another one.
Good data point.



From: "'g7jtt@...' g7jtt@... [ukmicrowaves]"
To: "uk microwaves@..."
Sent: Sunday, May 13, 2018 11:22 AM
Subject: Re: [ukmicrowaves] Re: Advice

 
Hi Kent thanks for a very informative reply I have one of your log periodic pcb antenna's so will give that a try. I see G4DDK does not stock your 1.5 to 15Ghz Vivaldi antenna, is there any likely hood of this changing ? If not will need to order directly . Also what is the power handling of the antenna's as I have around 50w on 23cm and 13cm whilst only 2w on 3cm.

All best John


On Sun, 13 May 2018 at 15:44, KENT BRITAIN WA5VJB@... [ukmicrowaves]
wrote:
 
Log Periodics do work well as a dish feed.
I agree, not quite as well as an optimized scalar feed, but the difference
is about 1 dB on a small dish.  And you get 3 or 4 ham bands.

The feedpoint of a dish is a theoretical point and usually calculated as though the
dish is used at optical frequencies.    In testing well over 100 dishes, the optimal
position for the feed is almost always closer to the dish than the calculated feed point.
In part this is because the feed is not an infinitesimal point.

Yes, the phase center of a log periodic does move, but it doesn't move very much.
When optimized at the highest frequency of use, the phase error at the lowest
frequency of use is quite minor.

Another option is using a Vivaldi antenna for the feed.& nbsp; I like to think of a Vivaldi
and a Ridged Horn without the horn.    Phase center is pretty constant with
frequency.   The Vivaldi has about 2 dB more gain than a Log Periodic and would
be more suitable for most offset fed dishes.

73  G8EMY/W5



From: "g7jtt@... [ukmicrowaves]"
To: ukmicrowaves@...
Sent: Sunday, May 13, 2018 7:44 AM
Subject:< /span> [ukmicrowaves] Re: Advice

 
just came across this site which has a multi band ring feed http://gvnweb.ddns.net:8081/dish1.html and links to some design software as well. the only thing is this is used on a prime focus dish are these feeds ok to use with an off set dish ?

cheers John 





Ole OZ2OE
 

 Hi all - I have been using one of Kent's Log Periodic antennas as feed for a 13+9 cm dish for some years now. As I run power on both bands (240W on 13cm and 80W on 9 cm, unfortunately only half at the feed) burnout of the PCB elements was a concern. I modified the antenna - a 2.1-5 GHz design - by soldering thicker wires on top of all the PCB tracks. This reduces the ohmic losses in the elements and the central feed structure (and improves cooling). To keep with the idea of the logarithmic antenna, I have used different diameter wires. See picture.

Adding the wires had the effect of raising the low frequency limit, so matching on 13 cm was compromised. I extended the rear 2 elements on each side by abt. 10 mm, until I had a reasonable return loss. This was done after the photo had been taken.

73 Ole OZ2OE

Den 13. maj 2018 kl. 19.00.21 +02.00, skrev 'jewell@...' jewell@... [ukmicrowaves] :

 

Now dealing with it....


Sam




On Sunday, May 13, 2018, 5:37 pm, KENT BRITAIN WA5VJB@... [ukmicrowaves] wrote:

 


Since I gave Sam an handful just a few weeks ago, he should have the 1.5-15's.
And at reduced performance they do work down to 800 MHz.

Wish I had enough power to test them to destruction.
50 watts of SSB or CW should be fine.   FM or one of the near CW digital modes
would be close, sort of depends on the length of your feedline.

BUT, it you cook it, let me know at what power level, and I will get you another one.
Good data point.



From: "'g7jtt@...' g7jtt@... [ukmicrowaves]"
To: "uk microwaves@..."
Sent: Sunday, May 13, 2018 11:22 AM
Subject: Re: [ukmicrowaves] Re: Advice

 
Hi Kent thanks for a very informative reply I have one of your log periodic pcb antenna's so will give that a try. I see G4DDK does not stock your 1.5 to 15Ghz Vivaldi antenna, is there any likely hood of this changing ? If not will need to order directly . Also what is the power handling of the antenna's as I have around 50w on 23cm and 13cm whilst only 2w on 3cm.

All best John



On Sun, 13 May 2018 at 15:44, KENT BRITAIN WA5VJB@... [ukmicrowaves]
wrote:
 

Log Periodics do work well as a dish feed.
I agree, not quite as well as an optimized scalar feed, but the difference
is about 1 dB on a small dish.  And you get 3 or 4 ham bands.

The feedpoint of a dish is a theoretical point and usually calculated as though the
dish is used at optical frequencies.    In testing well over 100 dishes, the optimal
position for the feed is almost always closer to the dish than the calculated feed point.
In part this is because the feed is not an infinitesimal point.

Yes, the phase center of a log periodic does move, but it doesn't move very much.
When optimized at the highest frequency of use, the phase error at the lowest
frequency of use is quite minor.

Another option is using a Vivaldi antenna for the feed.& nbsp; I like to think of a Vivaldi
and a Ridged Horn without the horn.    Phase center is pretty constant with
frequency.   The Vivaldi has about 2 dB more gain than a Log Periodic and would
be more suitable for most offset fed dishes.

73  G8EMY/W5



From: "g7jtt@... [ukmicrowaves]"
To: ukmicrowaves@...
Sent: Sunday, May 13, 2018 7:44 AM
Subject:< /span> [ukmicrowaves] Re: Advice

 
just came across this site which has a multi band ring feed http://gvnweb.ddns.net:8081/dish1.html and links to some design software as well. the only thing is this is used on a prime focus dish are these feeds ok to use with an off set dish ?

cheers John