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Author Topic: Better ERP?  (Read 1896 times)
N0URE
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Posts: 40




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« on: February 05, 2012, 01:03:12 PM »

It is time to move my vertical antenna from its temporary poor location to a better permanent position.
I operate HF QRP with some jumps to 100 watts with the K2. The bad news -- my antenna is an MFJ-1798, so I'm about -13db a good dipole.
I thought about a G5RV in the trees but that would only give me North-South and that is not what I need most and all the rain will cause me problems with the ladder line.

The Question ----  Can I gain much ERP if I get a remote antenna tuner positioned at the base of the antenna to replace the tuner in the shack?

At times I like to move down to the data/CW frequencies where my SWR is 3:1. My coax is 100 ft of the very low loss good stuff. Will my ERP improve or am I just moving the place where I make heat from the cable to the antenna? And what trade offs am I looking at if I put the tuner at the base where I have a good ground .vs. the feet point at the top? The MFJ-1798 is top feed with the ground plane on top.

Reading, the LDG-RT11 running on batteries would fit well but it is not made anymore. I don't know of other remote tuners that use latching relays to do well on batteries.
All suggestions will be appreciated.

73 John TI4/N0URE
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STAYVERTICAL
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Posts: 854




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« Reply #1 on: February 05, 2012, 01:51:19 PM »

Hi John,

I am not familiar with the MFJ1798, but having a look on the net, I get the idea it is a multiband antenna.
Assuming the antenna is set up to give reasonably low SWR on the bands you use, I guess the in shack tuner is mainly to tweak the swr to the best possible, not make up for large swr readings.

The feedline loss goes up with increasing swr, the type of feedline, as well as frequency.
If your swr is less than 3 to 1 on each band without a tuner, and you are using good quality feedline, I would say mounting the tuner at the antenna is not worth the trouble.
If you were using an antenna which had very high swr on some bands, then the feedline loss would be worth reducing by putting a tuner at the antenna, or tweaking the antenna.

For example, in my case, I use a random length of tubing for a vertical/groundplane and have a remote antenna tuner mounted at its base for tuning.
The reason is that if I didn't, the swr would be very high on just about every band, and the feedline losses would be pretty high.
I am using an FC40 remote random wire ATU, but it is specific to the Yaesu FT897D.
There are many good remote antenna tuners out there, but unless you have a really high swr on your feedline without one, it probably is not worth the inconvenience.

Hope this helps,

73s
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N0URE
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Posts: 40




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« Reply #2 on: February 05, 2012, 02:33:17 PM »

My biggest problem is that the bandwidth on 40 M is only 25K and on 80 M 35K. So if I want to move anywhere in the band I'm over 2 SWR and my 500 MW is going to heat.

John
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AA4PB
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Posts: 12701




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« Reply #3 on: February 05, 2012, 03:00:38 PM »

From what you said, you won't get any significant benefit by moving the tuner out to the base of the antenna. As long as the SWR on the feed line is under about 5:1 the coax loss won't be significant on frequencies up to 30MHz.

A dipole might help on 80M for signals out to several hundred miles. The vertical may perform better for signals beyond that. ERP is only part of the issue. What direction that energy is radiated (horizontally and vertical take-off) also needs to be considered. Signal to noise ratio is the determining factor on receive.
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WB6BYU
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Posts: 13046




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« Reply #4 on: February 05, 2012, 05:02:43 PM »

Quote from: N0URE

... my 500 MW is going to heat.



500 Megawatts!  I'm surprised the MFJ antenna hasn't melted at that sort of power level!



You can calculate the losses in the feedline using VK1OD's calculator here:

http://www.vk1od.net/calc/tl/tllc.php

Then you can see how much difference it makes running the feedline at 3 : 1 or 5 : 1.
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N0URE
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Posts: 40




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« Reply #5 on: February 06, 2012, 06:23:26 PM »

OK! small 'm'.

I checked 80 M. 

Mhz  SWR

3.5    2.5
3.53   1.3
3.6   4.1
3.7  6.0
3.8  6.8
3.9  6.9
4.0 6.8

I'll run the calculator for the 3.9 Mhz  and check for the difference between before cable .vs. other end.

John
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WB6BYU
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Posts: 13046




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« Reply #6 on: February 07, 2012, 11:16:41 AM »

Quote from: N0URE

...I thought about a G5RV in the trees but that would only give me North-South and that is not what I need most and all the rain will cause me problems with the ladder line.



The choice of a multi-band antenna depends (among other things) on the desired direction, distance
and frequency. 

The G5RV won't be particularly directive for closer stations on 80m and 40m, and it has a 6-lobed
pattern on 20m, with more lobes on the higher bands.  You can reduce the rain problems by coating
the twinlead with something like car wax that will encourage the rain to run off, or by using
old-fashioned ladder line (bare copper wires with occasional spacers) that doesn't have as much
insulation for the rain to affect.

If you have a path of particular interest, then you can design a wire antenna specifically for that
direction on one or two bands.

In any case, I'd strongly suggest putting up some sort of wire antenna - even if it is just a
dipole for one band.  That will give you a lot of improvement in some directions, and you can
always switch back to the other antenna for the bands / directions it doesn't cover as well.
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N4JTE
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Posts: 1154




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« Reply #7 on: February 07, 2012, 09:34:36 PM »

Good question, but I think you are confusing ERP with efficency of your antenna/ feedline/swr. Most times ERP is used to quote some kind of gain from a gain antenna, ain't happening with a single vertical no matter how situated.
Bob
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WB6BYU
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Posts: 13046




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« Reply #8 on: February 08, 2012, 11:04:50 AM »

Quote from: N4JTE
Good question, but I think you are confusing ERP with efficency of your antenna/ feedline/swr. Most times ERP is used to quote some kind of gain from a gain antenna, ain't happening with a single vertical no matter how situated.
Bob


No, ERP is Effective Radiated Power.  That is a function of transmitter power, feedline and other
losses, and antenna gain relative to some reference antenna.  So reducing the feedline losses
will increase ERP, as will increasing the transmitter power or improving the antenna.
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N4JTE
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Posts: 1154




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« Reply #9 on: February 08, 2012, 03:50:18 PM »

I understand that the ERP takes into account all system losses but I thought the antenna had to have Some real gain in order to measure anything using that term. In the question it seems that he was changing location which may or may not change his system's efficency but it still a non gain antenna. I use erp as a referance to differant antennas like a dipole compared to a two element beam which would double the ERP. Changing location of the vertical seems more of an efficency situaton than ERP unless someone convinces me that efficency is the same as ERP.
Respectfully,
Bob
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N4CR
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Posts: 1655




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« Reply #10 on: February 08, 2012, 04:58:38 PM »

Negative gain is real gain. We call it loss.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
WB6BYU
Member

Posts: 13046




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« Reply #11 on: February 08, 2012, 05:09:37 PM »

Quote from: N4JTE
...Changing location of the vertical seems more of an efficency situaton than ERP unless someone convinces me that efficency is the same as ERP.


So if his transmitter runs 100W, he has 3dB loss in his feedline, and his antenna is 13dB down from
a dipole, why wouldn't you say his ERP is 2.5 watts?  That makes it easy to compare to a QRP
station running 5 watts with 1dB of feedline loss and a beam with a gain of 7dBd, which would
have 20 watts ERP.

The point is the relative amount of power radiated.  It doesn't matter if you have losses in the
feedline, the antenna, or the ground around it:  anything that reduces the power delivered to
the antenna reduces the ERP.
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WA4FNG
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Posts: 162




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« Reply #12 on: February 10, 2012, 11:33:42 PM »

If you're only running 500mW, I'd be concerned with any type of loss betweem the transmitter and antenna. At that power you really want to maximize the antenna system as best you can for greatest efficiency. I really think that's the secret to really good results running QRPp.
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K1TWH
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Posts: 103




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« Reply #13 on: February 12, 2012, 08:49:24 AM »

John,
     I've been using RT-11's remotely for >5 years.   I use the MFJ bias-tee and place the operating voltage onto the coax.  Out at the antenna feedpoint, inside the RT-11, I make a .03 cap by placing three .01 1000V disc caps in parallel, in series with the input SO-239 to block the DC.   Then I pick off the DC with a 120uH choke which I purchased from Yaesu as a replacement for the choke they use inside the FT-857/897.    You can purchase the choke that MFJ uses inside their Bias-T a a replacement part.   I believe its a 100uH unit.  The RF choke is mounted to the center pin of the input SO-239 and the flying lead has a .01 cap to ground (RF bypass) and a wire to the +Vcc on the tuner chassis.    This delivers all the power the tuner needs, whenever it needs it.
      Tune up is as follows.   Find your frequency of interest or one nearby where you won't create interference to others,   apply low level RF (I use the AM mode) and then turn power on to the bias-T system.    Observe the RT-11 tuning.   When the SWR has been resolved, turn off power to the RT-11 at the bias-T.   You are now set to operate.
      Keep in mind that the max inductance is 20uH and the maximum shunt capacitance is 3400pF , so and RT11 has its limitations.  (An SGC SG-230 has 64uH max -L and atleast double the max capacitance.)
      Best regards,     Tom Howey  WB1FPA
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