15 Watt EIRP - What does that mean?

I am a bit surprised by the very strong signals of some fixed and portable 60m stations.
In other words, I have some doubts if the allowed maximum transmission power is respected by all 60m enthusiasts.

Therefore the question of whether it would be helpful for SOTA enthusiasts to publish a guidline for the maximum allowed transmit power in WATT to use with some typical antennas (e.g. dipole, quarter-wave vertical, short vertical, other)?

Any thoughts or proposals?

EDIT: This time the numbers are correct, I think

The UK limit is 100W at the transmitter and 200W eirp.

A dipole is 0dBd gain or 2.15dBi.
100W is 20dBW.
100W into a dipole is 22.15dBW or 164W eirp.

100W to a dipole is OK for the UK.

WRC-15 rules are 15W eirp and we’ll use the same dipole.

15W eirp is 11.8dbW eirp which is 9.6dBW into our dipole or 9.14W

Probably, allowing for feedline loss etc. 10W into a dipole is just, just legal for WRC-15 allocations.

If it is 15W EIRP, rather than 15dBW EIRP, I make that about 9W into a dipole…

2.15 dBi gain for a dipole gives a power ratio of about 1.6.

1.6 x 9 approximates to 15

I think!

Edit: mind you, I suppose that is a dipole in free space. I’m not sure how you account for environmental factors eg proximity of a wire fence acting as a reflector…?

or 10kw into a miracle whip…


Likewise for a Partridge Joystick VFA :wink:

Heinz @hb9bcb, I have had plenty of 59++ reports from people when I have been running 4.5W into an inverted V dipole on 5MHz in the UK. I don’t need to tell you it can be very difficult to tell how much power is being used remotely. Bringing the calculations and limits to people’s minds is probably a very good thing.

Especially a certain ON7 who is very active on JT65 and CW. On Saturday evening he was a full 30 dB stronger than any other station on a very crowded band!


Walt (G3NYY)

Not true, a dipole is up to ~9dBi depending on installation.

2.15dBi is the value for free space and ideal conditions, not for a real setup.


1 Like

One also needs to consider coax-loss. A long length of coax at home or use of higher loss (but thin and light) coax on a summit will reduce the actual power getting to the antenna and hence also the ERP and EIRP slightly.


Hi Razan,
Can you explain how a dipole can have as much as 9 dBi gain in a “real setup” - was that a typo and it should have been 0.9dBi?

Here’s what WikiPedia says - not that one can always trust WikiPedia!


E.g. Figure 4. The ground reflections are what give rise to the gain.

Expressing a regulatory power limit in EIRP is daft when the antennas to be used have gains that are affected by their environment. Fine for microwave systems, no good for low HF systems.

It is up to an administration to determine the maximum output power. The SOTA community does not need additional guidelines.
Let me tell you how WRC-15 decided on the 15 Watts. A number of countries, with the Russian Federation in the lead, was not in favour of a 5 MHz amateur allocation. This group could agree to a small allocation with a power limitation. There were many proposals, even one with a power level of 100 mW. A constructive French proposal said that a normal amateur radio has an output power of 100 Watts. Using digital modes an amateur would not use full power. So 50 - 60 Watts max. A wire antenna at low height will have a loss of 6 dB. So no complicated calculations. Just common sense.

73, Hans PB2T

Thanks for the document - those reflection factors depend on so many factors - as you say dBi is not a good measurement for HF especially when using simple antennas at low hights on different ground surfaces!

Extract (without diagrams):
Fig 12—Reflection factors for horizontal antennas at various heights above flat ground. The solid-line
curves are the perfect-earth patterns (broadside to the antenna wire); the shaded curves represent the
effects of average earth (k = 13, G = 0.005 S/m) at 14 MHz. Add 7 dB to values shown for absolute gain
in dBd referenced to dipole in free space, or 9.15 dB for gain in dBi. For example, peak gain over
perfect earth at 3/8 l height is 7 dBd (or 9.15 dBi) at 25° elevation.

Whilst the UK doesn’t have WRC-15 allocation we do have a max power and max eirp figures to work with.

You are right, but it’s the only number that is well known by amateurs. I can’t recall if dBd vs. dBi is included in UK amateur exams anymore but this number is known to many hams from knowing to check whether an antenna gain is be “boosted” by being given in dBi.

If you play the mind game of someone sat on a summit using 5MHz and suddenly someone from your licencing authority jumps out from behind a rock and asks “how do you know you are running a legal power?” you can at least show an attempt to comply by saying you assumed the free space figure. You may be running more than is legal due to the environment but demonstrating you have attempted to comply is what is important. You can then ask the licencing guy how much power can run from the radio and he’'ll scratch his head.

Andy, This is 2017. Licensing authorities have other things to do. :slight_smile:


Indeed, the concept of the “Ofcom mobile EIRP policing section” is amusing. You can see them having meeting before VHF field day.

Boss: “Right lots of overtime this weekend as it’s VHF field day and we know what that means.”

Staff1: “Crawling around tents and old caravans full of dangerous home brew PAs and EHT supplies?”

Staff2: “I hate field day, lots of old blokes who’ve been living on bacon sandwiches and beer and no showers for 2 days”

Staff3: “Boss, can’t we just pink ticket the lot of them in advance as we know they all run illegal powers?”


1 Like

Never known anyone to get such a thing, recently anyway !

Andy, the suggestive subject heading was chosen excactly for this reason. :wink:

Further I thought that especially technically not specially skilled SOTA colleagues would be grateful to see some practical guidelines on how to operate their station in compliance with the requirements laid down by the Administrations. :wink:

The issue you can’t easily know the real gain of the antenna is situ.

If you assume the dipole is 2.15 up on an isotropic antenna then 15W eirp is just under 10W from the radio. If it’s 9 then that’s a little under 2W. That is just the antenna. There will be other factors, coax loss etc. so the antenna could be much more lossy and higher powers would be acceptable.

If it was me, I’d run no more than 10W. There will always be someone who will run more and claim they have a particularly lossy setup whilst being the loudest signal on the band.

1 Like

[quote=“MM0FMF, post:13, topic:14453”]I can’t recall if dBd vs. dBi is included in UK amateur exams anymore but this number is known to many hams from knowing to check whether an antenna gain is be “boosted” by being given in dBi.[/quote]I’m pretty sure it was (though I don’t remember at which levels) when I took my exams in 2010/11, and it’s certainly something we teach in the courses at my local club. Whether the students remember what we teach… :wink:

When I took my RAE there was no mention of dBd, dBi, EIRP or even VHF, let alone UHF. Mind you, this was in 1962, the first year the symbol for a transistor appeared on the question paper! Two years later they started the “B” licence and I had a very, VERY steep learning curve!