EMRFD Message Archive 7088

Message Date From Subject
7088 2011-12-22 08:29:07 Ashhar Farhan Repeater front-end
I am undertaking homebrewing a vhf repeater for the local group. Will
people on this group suggest an easy front-end that will keep the
monster at +600khz away?
We shall, of course, use duplexers (until one of the mixer jocks
invents a front-end that can take the transmitter at the receiver
input). But I suppose there would be some front-ends suited to
repeater work?
- farhan VU2ESE

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7089 2011-12-22 09:49:17 kb1gmx Re: Repeater front-end
7094 2011-12-22 12:53:27 Mark Bayern Re: Repeater front-end
You're looking for a duplexer. From a 1972 article about building a
duplexer for a 2 meter repeater:

"Walk up to any ham who has already put a repeater on the air and ask
him, "What's the toughest technical problem you had with it?" He will
probably say that obtaining sufficient signal isolation between the
transmitter and receiver was the toughest. Satisfactory isolation can
be gotten, but always for a price. Many of the solutions to this
problem compromise receiver sensitivity or transmitter power output.
Other solutions throw off the balance between receiver and transmitter
coverage areas. When a duplexer is used, insertion loss is the
compromise. But any insertion loss is more than offset by the use of
one antenna for both the transmitter and receiver. Using one antenna
assures equal antenna patterns for both transmitting and receiving."

In 1973 (or was that '74?) we built one from the QST July 1972 article
and it was an excellent performer. The article is at
<http://www.repeater-builder.com/projects/2mduplexer.html>. Actually
if you start at <http://www.repeater-builder.com> you will find an
amazing amount off stuff. Most of it is very good. I see that another
ARRL publication recently had an article on building a duplexer for
the 440MHz band. QEX Sep-Oct 2009 issue.

Mark AD5SS





7099 2011-12-22 18:39:26 Ashhar Farhan Re: Repeater front-end
Allison, Mark,
Thanks for the inputs. I was just kidding about eliminating the
duplexers (with a nod to great stuff coming out from martein, chris,
caver, et al)..
The receiver as well as the transmitter are going to be crystal
controlled starting with low noise oscillators. But surely, we have
moved beyond the 70s in terms of receiver design?
Use of lower noise oscillators, double sheilding, etc. are some things
that I understand. What about the choice of mixers? The switches being
discussed following chris' paper don't work at 146MHz. Or do they?
For a person of my very modest capability and no commercial test gear,
aligning a front-end like the one in the triad is a challenge in any
case.
Helical resonators, followed by J310 rf amp, SBL-1 mixer, diplexer and
more J310 post IF amp is more like what I can handle. The local
oscillator will have to be an overtone oscillator followed by a
doubler or tripler.
I have seen a number of repeaters in other cities that just couple two
standard base stations together using separate antennae, in many
cases, both the antennae are mounted on the same tower, one below the
other. The coverage is not bad at all. The vijayawada repeater is
occasionally worked by hams in chennai (300 Km, over the sea).
These wild stories are usually true but unrepeatable(no pun).
I have pulled apart some of the newer gsm base stations. They are not
full duplex anyway. They are using active mixers. It was a big
surprise to me considering that there are several 50 watt transmitters
mounted on the shared towers which can tranmit without any
synchronization with each other.


On 12/23/11, Mark Bayern <plcmark@gmail.com> wrote:
> You're looking for a duplexer. From a 1972 article about building a
> duplexer for a 2 meter repeater:
>
> "Walk up to any ham who has already put a repeater on the air and ask
> him, "What's the toughest technical problem you had with it?" He will
> probably say that obtaining sufficient signal isolation between the
> transmitter and receiver was the toughest. Satisfactory isolation can
> be gotten, but always for a price. Many of the solutions to this
> problem compromise receiver sensitivity or transmitter power output.
> Other solutions throw off the balance between receiver and transmitter
> coverage areas. When a duplexer is used, insertion loss is the
> compromise. But any insertion loss is more than offset by the use of
> one antenna for both the transmitter and receiver. Using one antenna
> assures equal antenna patterns for both transmitting and receiving."
>
> In 1973 (or was that '74?) we built one from the QST July 1972 article
> and it was an excellent performer. The article is at
> <http://www.repeater-builder.com/projects/2mduplexer.html>. Actually
> if you start at <http://www.repeater-builder.com> you will find an
> amazing amount off stuff. Most of it is very good. I see that another
> ARRL publication recently had an article on building a duplexer for
> the 440MHz band. QEX Sep-Oct 2009 issue.
>
> Mark AD5SS
>
>
>
>
>
>
7103 2011-12-23 06:33:30 Thomas S. Knutsen Re: Repeater front-end
I think you would find that most design principles are a lot like what they
was in the 70's.
Passive high level mixers may be the way to go, if not avaible they can be
easy built. Just a couple of diodes in each leg or perhaps try the KISS
mixer approach?

Building the blocks as 50 ohm boxes with good screening may be the way to
go, then they can be aligned separate and it should work together.

I had an look inside an TAIT repeater. It's in basic an good screened
receiver, and an good screened transmitter with some logic between them. It
should be possible to use 2 standard transceivers and an good filter
between them. It may be an way to just get an repeater on air while you
experiment with the other.

73 de Thomas.

2011/12/23 Ashhar Farhan <farhanbox@gmail.com>

> **
>
>
> Allison, Mark,
> Thanks for the inputs. I was just kidding about eliminating the
> duplexers (with a nod to great stuff coming out from martein, chris,
> caver, et al)..
> The receiver as well as the transmitter are going to be crystal
> controlled starting with low noise oscillators. But surely, we have
> moved beyond the 70s in terms of receiver design?
> Use of lower noise oscillators, double sheilding, etc. are some things
> that I understand. What about the choice of mixers? The switches being
> discussed following chris' paper don't work at 146MHz. Or do they?
> For a person of my very modest capability and no commercial test gear,
> aligning a front-end like the one in the triad is a challenge in any
> case.
> Helical resonators, followed by J310 rf amp, SBL-1 mixer, diplexer and
> more J310 post IF amp is more like what I can handle. The local
> oscillator will have to be an overtone oscillator followed by a
> doubler or tripler.
> I have seen a number of repeaters in other cities that just couple two
> standard base stations together using separate antennae, in many
> cases, both the antennae are mounted on the same tower, one below the
> other. The coverage is not bad at all. The vijayawada repeater is
> occasionally worked by hams in chennai (300 Km, over the sea).
> These wild stories are usually true but unrepeatable(no pun).
> I have pulled apart some of the newer gsm base stations. They are not
> full duplex anyway. They are using active mixers. It was a big
> surprise to me considering that there are several 50 watt transmitters
> mounted on the shared towers which can tranmit without any
> synchronization with each other.
>
>
> On 12/23/11, Mark Bayern <plcmark@gmail.com> wrote:
> > You're looking for a duplexer. From a 1972 article about building a
> > duplexer for a 2 meter repeater:
> >
> > "Walk up to any ham who has already put a repeater on the air and ask
> > him, "What's the toughest technical problem you had with it?" He will
> > probably say that obtaining sufficient signal isolation between the
> > transmitter and receiver was the toughest. Satisfactory isolation can
> > be gotten, but always for a price. Many of the solutions to this
> > problem compromise receiver sensitivity or transmitter power output.
> > Other solutions throw off the balance between receiver and transmitter
> > coverage areas. When a duplexer is used, insertion loss is the
> > compromise. But any insertion loss is more than offset by the use of
> > one antenna for both the transmitter and receiver. Using one antenna
> > assures equal antenna patterns for both transmitting and receiving."
> >
> > In 1973 (or was that '74?) we built one from the QST July 1972 article
> > and it was an excellent performer. The article is at
> > <http://www.repeater-builder.com/projects/2mduplexer.html>. Actually
> > if you start at <http://www.repeater-builder.com> you will find an
> > amazing amount off stuff. Most of it is very good. I see that another
> > ARRL publication recently had an article on building a duplexer for
> > the 440MHz band. QEX Sep-Oct 2009 issue.
> >
> > Mark AD5SS
> >
> >
> >
> >
> >
> >
7104 2011-12-23 09:21:56 ha5rxz Re: Repeater front-end
Make friends with your local Army depot.

Back in the 1970's I helped build a repeater for what was then my local ham radio club. When it came to constructing the front end filters we used six inch diameter shell casings which were wonderfully made from solid brass. We just paid the scrap value for twelve casings, eight on receive and four
7105 2011-12-23 10:38:48 Ashhar Farhan Re: Repeater front-end
Haha, that's some story. I guess you had a blast with the duplexers
(harr! Harr!)
All the vhf duplexer designs tune the cavity with rods changing their
effective length by telescoping. Is it also possible to tune the
quarter wave cavity by means of a disc capacitor? I know that
striplines can be tuned that way. What about the quarter wave
cavities?

On 12/23/11, ha5rxz <ha5rxz@gmail.com> wrote:
> Make friends with your local Army depot.
>
> Back in the 1970's I helped build a repeater for what was then my local ham
> radio club. When it came to constructing the front end filters we used six
> inch diameter shell casings which were wonderfully made from solid brass. We
> just paid the scrap value for twelve casings, eight
7106 2011-12-23 10:48:32 Thomas S. Knutsen Re: Repeater front-end
Would not capacitors lower the Q of the resonator?


2011/12/23 Ashhar Farhan <farhanbox@gmail.com>

> **
>
>
> Haha, that's some story. I guess you had a blast with the duplexers
> (harr! Harr!)
> All the vhf duplexer designs tune the cavity with rods changing their
> effective length by telescoping. Is it also possible to tune the
> quarter wave cavity by means of a disc capacitor? I know that
> striplines can be tuned that way. What about the quarter wave
> cavities?
>
>
> On 12/23/11, ha5rxz <ha5rxz@gmail.com> wrote:
> > Make friends with your local Army depot.
> >
> > Back in the 1970's I helped build a repeater for what was then my local
> ham
> > radio club. When it came to constructing the front end filters we used
> six
> > inch diameter shell casings which were wonderfully made from solid
> brass. We
> > just paid the scrap value for twelve casings, eight
7107 2011-12-23 11:33:20 kb1gmx Re: Repeater front-end
7108 2011-12-23 14:30:45 kb1gmx Re: Repeater front-end
7113 2011-12-27 19:53:36 Ashhar Farhan Repeater front-end
Sorry, this was meant for the group but went just ti jim. Blackberry
is difficult to operate with snow gloves!
- farhan

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