My Glomex Antenna System: Telly Tech Q's

[Dr8ton]

The following is my recently fitted Narrowboat TV and radio solution (12 volt supply):

 

A single omnidirectional Glomex antenna feeds a Meos TV and Kenwood car radio at the same time with gain added (via a Glomex gain controller) when reception is difficult or if I can't raise the antenna sufficiently.

 

Questions:

 

Does signal strength reduce each time a coaxial cable is joined? I have 4 joins on mine.

 

Does signal strength reduce significantly with a longer coaxial cable run? Mine is about 60 feet.

 

Roughly how much of the signal is leaked by the coaxial connectors and is it worth buying only gold plated ones?

 

Does it matter much that we have joined 75 Ohm cable to a very short run of inaccessible "pre-existing" (brown) coaxial cable from the boat's analogue TV days?

 

How any mA (typically and very approximately) will my 12V Glomex gain controller draw on full gain? The tech. spec. cites 25 mA but for what I don't know (the gain controller model number is at the end of this post).

 

 

If you want the background read on.......

 

Thanks,

Dr8ton

 

 

The Glomex omnidirectional antenna is on an extending pole mounted on the outside of my cratch board.

 

Seventy five Ohm cable carries the signal (via a 2-way cable splitter) to a Meos combi TV and to a Kenwood car radio. The total cable run is about 60 feet (though roughly 45 feet is currently coiled up so I can move the antenna about for testing purposes).

 

On the antenna side of the splitter is a 12V Glomex gain controller. Note that my gain controller does not have a by-pass built in so it must be switched on to receive a radio and/or a TV signal.

 

Yesterday I tested the finished system with the boat static on her mooring in a good reception area. After raising and lowering the antenna and messing about with the controller (e.g with the antenna lowered to cabin roof height I need to add about 25% gain) I got a strong signal on all Freeview channels and excellent FM from the radio, even at the same time.

 

Then, as an experiment to test the effect of the cable joins, I simply plugged the Glomex antenna directly into the TV. I found I could lower the antenna completely and even bring it inside the cratch with no significant loss in signal quality. So on the mooring at least I do have this option (i.e. after rigging a gain controller bypass for convenience).

 

That said, my simple experiment suggests that splitting the cable (e.g. to incorporate the gain controller) reduces signal quality. To compensate I wound up the gain (at the expense of mA). Of course this wasn't the point of fitting the gain controller! It's main function will be to help improve poor reception "on the move". Even so, I can't help wondering if splitting the cable to accommodate a gain controller is counter-intuitive, at least in part.....

 

 

Equipment used:

 

Antenna: Glomex V9125

 

12/24V Gain controller: Glomex 50023 / 98EC (with on / off switch wired to 12V panel)

 

Cable: 75 Ohm (supplied by Glomex as part of kit)

 

Two-way splitter: Glomex V9147

 

Coaxial connectors: F type

 

TV: 19" Meos TV/DVD combi 192B (plugged into 12V DC socket)

 

Radio: Kenwood Radio/CD/Aux KDC 24

[nicknorman]

Q1: yes, a bit, due to impedance mismatch within the connector.

 

Q2: yes, but it varies according to the quality of the cable.

 

Q3: hard to quantify, but gold plating the contacts will make no difference. Connectors are gold plated because the gold doesn't corrode and hence low voltage low frequency (eg audio frequency) signals are not affected by a thin layer of oxide that might form on lesser connectors. At TV RF any slight gap due to an oxide layer is easily bridged by capacitance.

 

Q4: not much. We have already established that each connection introduces slight losses /reflections due to impedance mismatching, but presuming the number of connections remains the same, having a bit of old style coax as opposed to new coax, presuming it is the correct impedance, won't make any significant difference.

 

Q5: 25mA. I doubt it will make any significant difference what the gain is set to. It's not like a power amp where turning up the volume (and thus the output power) increases the power demand.

Edited July 13, 2014 by nicknorman

[Dr8ton]

Thanks a lot Nick for answering all my questions.

 

You have explained the observed difference in signal strength when I plug the antenna directly into the TV.

 

Also very useful to learn that the gain controller draw is nominal even on full gain. This means I don't have to worry about a bypass.

 

Best,

Dr8ton

[smileypete]

In theory the amp should be able to make up the losses from splitter and cable and much more, but in practice the amp does add some noise which degrades the signal. A passive splitter will drop the signal level by half (-3dB) which sounds a lot but isn't toooo bad.

 

There some good info on cable types and all other stuff on this site:

 

http://www.wrightsaerials.co.uk/articles/coax-cable-quality.shtml

 

A typical good system would be satellite cable from TV to outside, then foam filled satellite cable from there to the aerial, possibly with an extension too. BNC to F type adapters can make it easy to connect things quickly.

 

ETA: This site is worth a look too:

 

http://www.satcure.co.uk/tech/amps.htm

 

cheers, Pete.

~smpt~

Edited July 15, 2014 by smileypete

[Dr8ton]

Hi Pete,

 

The 2-way splitter is passive. On my mooring with 25% gain and the antenna slightly raised it is possible to have the radio and TV on together with no static or picture break-up (with less than 25% gain I get zip, except if I raise the antenna). Unexpectedly, with the radio on standby or even unplugged I still need about 25% gain for the telly - not sure why. Anyway at 25 mA, winding up the gain is no problem so on the mooring it's good. But what about on the move?

 

Well after a quick read of your links (brilliant!) I am learning that there might be a few ways to compensate for signal loss should the gain controller + antenna extension fail to cope:

 

1. Use lower loss cable - I think mine is "type C" with a solid dielectric but I'll check when aboard

 

2. Reduce the cable length between the antenna and the gain controller to about 15 feet (all I need)

 

3. Plug the antenna directly into the gain controller (at present there is a coax wall plate in-between)

 

Also, should on-board interference become an issue, I can replace the boat's "pre-existing" coaxial wall plates: between the antenna and gain controller, between the gain controller and TV and between the splitter and radio.

 

 

When the signal is too strong I will simply wind gain off manually (the amp is not automatic so won't work with an attenuator???).

 

Thanks,

 

Dr8ton

[smileypete]

If you have the old style TV aerial wall plates, these can be quite lossy, so could be good to replace with 'F' connector wall plates normally used for satellite, and screw-on F plugs on the cable, this should give minimal loss at the connections.

 

Also well worth using decent cable, RG6 should be OK though a little cheapo, the (xx)100 stuff is a cut above and worth paying a bit more for, this looks like a decent price for 30m:

 

http://www.ebay.co.uk/itm/Satellite-Digital-Coaxial-Cable-Philex-30m-F-Plugs-PF100-FULLY-Screened-aerial-/121382850710

 

cheers, Pete.

~smpt~

Edited July 16, 2014 by smileypete

[Dr8ton]

Good morning Pete,

 

Yes - the wall plates are the old style ones. I'll fit modern ones as you suggest. I've got loads of F connectors.

 

Cheers for the cable link.

 

Dr8ton